The industry term for these kinds of businesses is "MVNO" - mobile virtual network operator. Wikipedia has a reasonably current list of the ones currently operating in the USA at https://en.wikipedia.org/wiki/List_o...work_operators. It's a hard list to keep current though; setting up an MVNO isn't hard, and there are lots of them.

Most of them, though, don't switch carriers dynamically; your phone is set to use one of them, period. Google Fi (and maybe others by now) will switch depending on signal strength.