Add a RTE field to tt_content

tt_content.php

<?php
// typo3conf/ext/yourext/Configuration/TCA/Overrides/tt_content.php

use TYPO3\CMS\Core\Utility\ExtensionManagementUtility;

$fields = [
    'my_rte' => [
        'label' => 'Mein RTE',
        'config' => ['config' => [
            'type' => 'text',
            'eval' => 'required,trim',
        ],
    ],
];
ExtensionManagementUtility::addTCAcolumns('tt_content', $fields);

// Define the CTypes as comma separated list, to which you want to add your field
ExtensionManagementUtility::addToAllTCAtypes('tt_content', 'my_rte;;;richtext[]:rte_transform[mode=ts];', 'text,textimage');