Rich text editors in the TYPO3 backend 

When you configure a table in $GLOBALS['TCA'] and add a field of the type text, which is edited by a <textarea> , you can choose to use a rich text editor (RTE) instead of the simple form field. An RTE enables editors to use visual formatting aids to create bold and italic text, paragraphs, tables and more.

For full details about setting up a field to use an RTE, please refer to the chapter labeled 'special-configuration-options' in older versions of the TCA Reference.

The short story is that it's enough to set the key enableRichtext to true.

<?php

return [
  //...
  'columns' => [
    'rte_5' => [
      'label' => 'RTE Example',
      'config' => [
        'type' => 'text',
        'enableRichtext' => true,
        'richtextConfiguration' => 'full',
      ],
    ],
  ],
];

This works for FlexForms too:

<poem>
  <label>RTE Example (Flexform)</label>
  <config>
    <type>text</type>
    <enableRichtext>true</enableRichtext>
    <richtextConfiguration>full</richtextConfiguration>
  </config>
</poem>