What happens to US license when moving to Canada?

Not sure what you mean by "what happens to" your (US) license (??) -- it remains active until its expiration date, and then you can renew it if you choose, the same as if you were in the US, if you have maintained a legal residence in your home state. It doesn't self-destruct when you cross the border, or anything like that ...

You do not have to maintain a legal residence in a state to maintain a license, you just need to pay the renewal fee.

You're right, of course -- just a temporary brain freeze.

It's no different than holding licenses in more than one US state -- they don't cancel each other out; you can hold as many licenses as you're willing to pay for , and they are all valid.