How to make my own Assignees field?

I can see the field on Tasks in various templates (e.g. Scrum for Teams), but I can’t find a way to add it myself to a database I want.

I can’t find Assignees in the list of field types when I press “+New Field”, it doesn’t seem to be a relation (if I set up a relation with Users, the icon is different), and I can’t edit the Assignees field predefined in various templates, only delete it.

I’ve searched the docs and the internet for any clues, and the only I could find is that it’s some kind of “Assignment extension” (someone mentioned it on this forum), but this clue didn’t led anywhere further.

You should find ‘Assignments’ in the list of options when you choose to add a new field.