Migration from CKEditor 4
Differences between 4 and 5. Plugin compatibility table.
The CKEditor module allows placing separators and wrappers in the toolbar.
If there are no separators, they are automatically grouped when overflowing.
See the full options here: https://ckeditor.com/docs/ckeditor5/latest/getting-started/setup/toolbar.html
You can edit the config file and add dropdown menus using the advanced configuration format.