Generally it's only a thing any more if you get your phone discounted as part of signing up with or renewing at a certain carrier.

If you just go out and buy a phone outright you can generally use it anywhere.

I think it is, but it's not like you can't get unlocked iPhones in the US. I think carriers also are required to unlock your phone if you ask them, but I may be imagining that.

It is. I paid discounted full price through my employer and because it’s on a business plan it’s locked.

If you buy from a carrier, yes.