I feel like someone's gonna correct me

Mike was experimented on but not in fnaf 4

YES
OMG FINALLY
like I get the stuff about Mike being experimented on and I don’t deny that it probably happened, but the thing that CC saw WAS HIS NIGHTMARES.

I think.it was both

That’s what I say too

Because remember, we see cc alone in minigames, while mike was absent.

And mike drew nightmare fredbear

I think there's evidence towards a bit of both. Mike's brother definitely had the FNAF 4 nightmares first, either or experiments or just from the pure unbridled fear of a child, likely both. All of this makes sense again due to his fear of the animatronics, and the fact one of them bit him in the head, which explains all the mouth and teeth imagery in the designs. But later on, Mike would have the same nightmares too, except his are slightly different (e.g. the phone guy in the background).