Are a large part of the people living in a city the kind of semi-transitory-but-also-there-for-years people you describe?

I'd wager that's a small proportion of almost every city. Most cities will have tourists who are visiting for a few days or weeks, and long term residents who have a permanent address there. The percentage of people living full time in hotels or airbnbs must be tiny. Perhaps in high cost of living cities there's more "hidden homeless" living on couches, but even then it's not going to be "a large part" of a city.

Among those that are registered to vote locally, most don't. Regardless of whether or not people should or shouldn't be able to vote, many of those currently with the ability to do not.