Extra editor on top of default in Custom Post Type

There are plugins available, such as Advanced Custom Fields, that allow you to add additional content areas. You set up the fields, and choose what post types they will appear on, and then you’ll be able to enter information in a WYSIWYG editor basically like the Classic Editor.

ACF saves the data to postmeta rather than post_content, so you then need to adjust your theme templates to specifically call for that postmeta wherever it is needed.