I think this is mostly a Wireguard thing and not specifically a Tailscale thing. Wireguard does what they call "cryptokey routing" where if you prove you possess a key that the other peer knows, you get the traffic (subject to firewall, allowed IPs list, etc etc). Wireguard stores the most recent address:port that it heard from a particular cryptokey on, but it natively lets peers roam, as long as only one roams at a time.