No Stupid Questions

First, fuck body shaming. What someone looks like has no bearing on their rights under law, nor whether or not the can be nude when legal. That's shitty thinking, and you should reconsider your mindset.

That out of the way...

Highly location dependent. Hell, afaik, there aren't countries that have nudity laws. I could be wrong, but I've not run across it despite being interested in such laws

Here in the US, that kind of thing would be jurisdiction bases. City and/or county (or parish) level. I think some states have laws about it, but I can't be arsed to look up which ones at 2 in the morning. I think there's stuff on the books when you're on federal land, but damned if I can recall the details. But there's no blanket law that covers the entire country.

However, the public nudity laws as regard this question i've seen tend to be based around when a person has an expectation of privacy. Generally, you can be naked in your house, and fuck anyone trying to peek in the window. But once you open the curtains, you're in public view for this kind of thing. You also run into specifics of what is and isn't nudity. Some places don't allow women's titties, but do men's. Some places are less strict about anything but genitals.

Just playing off of memory though, most places in the US would expect you to at least cover your genitals when answering the door.