To get the categories from inside the editor of a post you can make use of the following selectors:
The categories the post has in the published version:
wp.data.select("core/editor").getCurrentPostAttribute("categories")
The current categories of the edit (for example if the user has selected a new category but hasn’t saved the post it will appear with this selector but not with the former):
wp.data.select("core/editor").getEditedPostAttribute("categories")
This will give you an array with the id of each category.