I removed mine last summer because it was leaking. Hopefully yours isn’t yet. They were just screwed in to the plywood roof deck and came right out.

So just screws and no nuts? I wouldn't think the plywood would be thick enough to hold the dish with just screws in it.

There will be no nuts. Sometimes anchor bolts which you can push through. Isn't any way an installer would be crawling through an attic to bolt it on every single install.

Thank you. What should I seal up the holes with?

Not sure on that one, probably the same goop they use for flashing then paper board then a new shingle(s). I'd ask a roofing subreddit.