config_form.php

<?php
// use a config in another module
$config = \Drupal::config('my_custom_module.my_config');
$var_value = $config->get('var_name');

Drupal8FormAPICode.php

<?php

public function buildForm(array $form, FormStateInterface $form_state) {

    // If the form was submited the form build has access to the user entered data.
    $user_input = $form_state->getUserInput();
    $has_been_submitted = !empty($user_input);
    
    // multiple submit buttons
    $form['cases']['submit'] = [
      '#type' => 'submit',
      '#name' => 'submit',
      '#value' => $this->t('Submit'),
    ];

    if ($has_been_submitted) {
      $form['cases']['reset'] = [
        '#type' => 'submit',
        '#name' => 'result',
        '#value' => $this->t('Clear Form'),
      ];

    }
    

}


public function submitForm(array &$form, FormStateInterface $form_state) {
  // Get all values.
  $values = $form_state->getValues();
  
  // Get a single value.
  $form_state->getValue('product_id');
    
  // check what button was clicked
    $action = $form_state->getTriggeringElement()['#name'];
    \Drupal::messenger()->addMessage('$action=' . $action);
    
 // Set the form to rebuild - othrwise all values entered will be lost.   
 $form_state->setRebuild();    
    
}

Drupal8FormAPIElements.php

<?php
  public function buildForm(array $form, FormStateInterface $form_state) {
    
    // See list of element types: https://api.drupal.org/api/drupal/elements/9.0.x
    // see parant FormElement class https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Render%21Element%21FormElement.php/class/FormElement/9.0.x
    
    $form['fieldset_1'] = array(
      '#type' => 'fieldset',
      '#title' => $this->t('fieldset 1'),
    );
    
    // Collapsible fieldset - Replaced with HTML5 details elements
    $form['advanced'] = array(
      '#type' => 'details',
      '#title' => t('Advanced settings'),
      '#open' => FALSE, 
    );
    
    
    // Text field
    $form['text_field'] = [
      '#type' => 'textfield',
      '#title' => $this->t('Name'),
      '#description' => $this->t('Enter your name.'),
      '#maxlength' => 50,
      '#size' => 50,
      '#default_value' => 'Guy',
      '#required' => TRUE,
      '#disabled' = FALSE;
    
    ];    
    
    // password field
    $form['password'] = [
      '#type' => 'password',
      '#title' => $this->t('Your password'),
      '#description' => $this->t('Please enter your password'),
      '#maxlength' => 25,
      '#size' => 25,
      '#weight' => '0',
    ];
    // with config setting
    $form['password'] = [
      '#type' => 'password',
      '#title' => $this->t('Your password'),
      '#description' => $this->t('Please enter your password'),
      '#maxlength' => $config->get('pg_catalogue_relation_type') ?? 25, 
      '#size' => 25,
      '#weight' => '0',
    ];


    // number
    $form['line_number'] = [
      '#type' => 'number',
      '#title' => $this->t('Line number'),
      '#weight' => '10',
    ];
    
    // Markup
    $form['markup_field']  = [
      '#markup' => '<H1>This is a bit of HTML</H1>',
    ]
    
    $form['checkbox_field'] = [
      '#type' => 'checkbox',
      '#title' => $this->t('Tick to agree to T&Cs.'),
      '#default_value' => $log_all_requests,
    ];
    
    // Select
    $form['options'] = [
      '#type' => 'select',
      '#title' => $this->t('Options'),
      '#options' => [1 => 'one', 2 => 'two', 3 => 'three'],
      '#default_value' => 1,
    ];
   // Options for an hierarchical select 
      '#options' => [
        '1' => t('One'),
        'Two' => [
          '2.1' => t('Two point one'),
          '2.2' => t('Two point two'),
        ],
        '3' => t('Three'),
      ],
    
    
    
    // Hidden field - will show in the source of the form but will not be displyed.
    $form['hidden_id'] = array(
      '#type' => 'hidden',
      '#value' => 123,
    );
    
    // Value - esosated with the form but will not be in the source of the page.
    $form['entity_id'] = array(
      '#type' => 'value',
      '#value' => $entity_id,
    );    
    
    
    // Simple mark up
      $form['head']  = [
        '#markup' => '<H2>This is an H2</H2>',
      ];
    
    
    // Advance markup using item
      $form['html_with_label']  = [
        '#type' => 'item',
        '#title' => $this->t('Recipe'),
        '#description' => $this->t('The recipe to base the new product on'),
        '#markup' => $recipe_name,
        '#prefix' => '<div id="field-recipe-name">',
        '#suffix' => '</div>',
      ];
    
    
    
    // Submit
    $form['submit'] = [
      '#type' => 'submit',
      '#value' => $this->t('Submit'),
    ];

    // Date time - Time only
    $form['import_time'] = [
      '#type' => 'datetime',
      '#title' => $this->t('Import time'),
      '#date_date_element' => 'none',
      '#date_time_format' => 'H:i:s',
      '#date_time_element' => 'HTML Time',
      '#default_value' => $config_import_time,
    ];
    
    // Date field
    $form['archive_date'] = [
      '#type' => 'date',
      '#title' => $this->t('Archive date'),
      // Default to 1 year ago.
      '#default_value' => date('Y-m-d', time() - 31536000) ,
    ];
    
    // Date list - example showing - Year set as text all other elements use dropdown 
    $form['datelist_element'] = [
      '#title' => 'datelist test',
      '#type' => 'datelist',
      //'#default_value' => new DrupalDateTime('2000-01-01 00:00:00'),
      '#date_part_order' => [
        'month',
        'day',
        'year',
        'hour',
        'minute', 'ampm',
      ],
      '#date_text_parts' => ['year'],
      '#date_year_range' => '2010:2020',
      '#date_increment' => 15,
    ];    
    
    $form['datetime_element_html'] = [
      '#title' => 'HTML Datetime',
      '#type' => 'datetime',
      '#date_date_format' => 'Y-m-d',
      '#date_time_format' => 'H:i:s',
      '#date_date_element' => 'HTML Date',
      '#date_time_element' => 'HTML Time',
      '#date_increment' => 1,
    ];
    
$form['author'] = array(
  '#type' => 'fieldset',
  '#title' => $this->t('Author'),
  '#attributes' => ['class'  => ['tile', 'tile--subtle', 'tile--rounded']],
  
);
$form['author']['name'] = array(
  '#type' => 'textfield',
  '#title' => $this
    ->t('Name'),
);    
    
// Conditional fields
$form['colour_select'] = [
      '#type' => 'radios',
      '#title' => $this->t('Pick a colour'),
      '#options' => [
        'red' => $this->t('White'),
        'other' => $this->t('Other'),
      ],
    ];
   $form['custom_colour'] = [
      '#type' => 'textfield',
      '#size' => '60',
      '#placeholder' => 'Enter favourite colour',
      '#states' => [
        'visible' => [
          ':input[name="colour_select"]' => ['value' => 'other'],
        ],
      ],
    ];    

  // Add CSS. 
    $form['#attached']['html_head'][] = [
      '#tag' => 'style',
      '#value' => '.content__inner ul  { text-align: left; }' ,
    ];
    
    
    
    return $form;
  }

field_grouping.php

<?php


// vertical tabs - https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Render%21Element%21VerticalTabs.php/class/VerticalTabs/
$form['information'] = array(
      '#type' => 'vertical_tabs',
      '#default_tab' => 'edit-publication',
    );
    $form['author'] = array(
      '#type' => 'details',
      '#title' => $this
        ->t('Author'),
      '#group' => 'information',
    );
    $form['author']['name'] = array(
      '#type' => 'textfield',
      '#title' => $this
        ->t('Name'),
    );
    $form['publication'] = array(
      '#type' => 'details',
      '#title' => $this
        ->t('Publication'),
      '#group' => 'information',
    );
    $form['publication']['publisher'] = array(
      '#type' => 'textfield',
      '#title' => $this
        ->t('Publisher'),
    );

form_alter.php

<?php

/**
 * Implements hook_form_alter().
 * Template
 */
function my_module_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  \Drupal::messenger()->addMessage("form_id = $form_id", 'status');
  if ($form_id == 'xxxxxx') {    
    // Do stuff
  }
}

/**
 * Modify the address of a contact us form.
 */
function my_module_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  if ($form_id == 'contact_message_contact_us_form') {
    $available_country_codes = ['GB','US','FR'];
    $form['field_contact_country']['widget'][0]['value']['#available_countries'] = $available_country_codes;

    $form['field_contact_country']['widget'][0]['value']['#default_value'] = 'GB';
  }
}


// After build - Template
function my_module_form_alter(&$form, FormStateInterface $form_state, $form_id) {
    $form['#after_build'][] = 'my_module_after_build';
    // Add a validate  
    $form['#validate'][] = 'my_module_form_validate';
    // Add a submit that will run before the other submits
    array_unshift($form['actions']['submit']['#submit'] , 'my_module_form_submit');
}

function my_module_after_build($form, &$form_state) {
  // do stuff to the form.
  return $form;
}

function my_module_form_validate(array $form, FormStateInterface $form_state) {
  $values = $form_state->getValues();
  $selected_country = $form_state->getValue('field_x');

}


function  my_module_form_submit(array $form, FormStateInterface $form_state) {

}

form_conditional_fields.php

<?php

// List of statuses that can be set
visible
invisible
required
enabled
disabled

// Condition types

  // Checkbox
  ['checked' => TRUE]

  // Select
['value' => 'option key']  
  
  
// Or conditions
        '#states' => [
          'disabled' => [
            [':input[name="checkbox1"]' => ['checked' => TRUE]],
            'or',
            [':input[name="checkbox2"]' => ['checked' => TRUE]],
          ],
        ],              
  
// And conditions
        'required' => [
          [
            [':input[name="checkbox1"]' => ['checked' => TRUE]],
            [':input[name="checkbox2"]' => ['checked' => TRUE]],
          ],

// Multiple actions
      '#states' => [
        'visible' => [
          [
            [':input[name="checkbox1"]' => ['checked' => TRUE]],
          ],
        ],
        'required' => [
          [
            [':input[name="checkbox2"]' => ['checked' => TRUE]],
          ],
        ],
      ],
          



// Examples  
   // Controlled by a checkbox
    $form['toggle_disable'] = [
      '#type' => 'checkbox',
      '#title' => $this->t('Override last run'),
      '#default_value' => FALSE,
    ];

    // Fieldsets can be used to controll the state of its elements 
    // also useful if a type like datetime #states' does not work for some reason.
      '#type' => 'fieldset',
      //'#title' => $this->t('Fieldsset 1'),
      '#states' => [
        'disabled' => [
          ':input[name="override_last_run"]' => ['checked' => TRUE],
        ],
      ],

      '#type' => 'fieldset',
      //'#title' => $this->t('Fieldsset 2'),
      '#states' => [
        'enabled' => [
          ':input[name="override_last_run"]' => ['checked' => TRUE],
        ],
      ],

    );