I've seen people on here confirm that if you adopt a villager with one of the five starter houses their house gets upgraded to their usual one when they move out, but does the same apply to regular houses?

like for example I have Judy with her nice pastel block layout with star lights, but she constantly asks to buy ugly fish and bugs from me. if I ever adopted her out, would she still have all the ugly creatures in her house when she moves?

I've gotten two villagers from other people and both of them had pristine houses when they moved in, but they were surprise move-ins and not intentional adoptions so idk what condition their houses were in before

Agent S moved in and I'm pretty sure she was on my niece's island. She doesn't  have any fish or bugs and I'm pretty sure she did when she lived on my niece's island. I think her house changed too.

if her house changed too, then she might've been one of your niece's 5 starting villagers, since the first 5 villagers you get always have the same house, but they change to their proper unique house once they move out. so unfortunately that probably doesn't answer the question

I don't think they reset. I recently adopted a villager and they did has a fish out in their house when they unpacked, so I think they set it up however it was when they moved out.

2nd island picked up Stu from Island 1.  Stu was wearing the shirt Island 1 gave him