Yes, many start on mastodon.social and then find an instance more reflective of their community. Importing and exporting users gets to be a pain.

But you can make your own Domain your Fediverse instance with a bit of work.

As many as you want. I have accounts on mastodon.social I am also @jgmac1106@stream.jgregorymcverry.com and jgmac1106@jgregorymcverry.com I have account on scholar.social but they don’t like people publishing from their own domain