The system extension is part of common Composer distributions such as typo3/cms-base-distribution. It is activated by default and ships with a default configuration for editors.
If you don't use any distribution the system extension can be installed via:
composer require typo3/cms-rte-ckeditor
For legacy installations that do not use Composer, EXT:rte_ckeditor is already part of the distributed package.
Activate the RTE CKEditor extension in Admin Tools > Extension Manager.