Code sample from https://wptips.dev/global-var-n-filters-in-timber/‎

timber.php

class MyTimber extends TimberSite {

  function __construct() {
    add_filter( 'timber_context', [$this, 'add_to_context'] );
    add_filter( 'get_twig', [$this, 'add_to_twig'] );
    parent::__construct();
  }
  
  // Add Global variable
  function add_to_context( $context ) { 
    // Site's settings like site.title
    $context['site'] = $this; 
    $context['home_url'] = home_url();

    // Menu - Timber stored it as an array so you can loop it and write custom markup
    $context['nav'] = new TimberMenu( 'main-nav' );

    // Link to the image inside your theme
    // Example:   <img src="{{ images }}/logo.svg">
    $context['images'] = get_template_directory_uri() . '/images';

    // Footer Widget - for post widget, no need to be global
    $context['footer_widgets'] = Timber::get_widgets( 'footer_widgets' );

    // ACF Options Page, if you have one
    if( function_exists( 'acf_add_options_page' )) {
      $context['options'] = get_fields( 'options' );
    }

    // WooCommerce, if installed
    if( class_exists('WooCommerce') ) {
      global $woocommerce;
      $context['woo'] = $woocommerce;
    }

    return $context;
  }

  function add_to_twig( $twig ) {
    $twig->addFilter( new Twig_SimpleFilter( 'my_example', [$this, 'filter_my_example'] ) );
    $twig->addFilter( new Twig_SimpleFilter( 'my_example2', [$this, 'filter_my_example2'] ) );
    return $twig;
  }


  /**
   * Basic custom filter
   *
   * {{ post.content | my_example }}
   */
  function filter_my_example( $value ) {
    // do something
    return $value;
  }

  /**
   * Has passed argument
   *
   * {{ post.content | my_example2( 'arg' ) }}
   */
  function filter_my_example2( $value, $arg ) {
    // do something
    return $value;
  }
}

new MyTimber(); // call the class