You navigate from your local instance. So like you just use hyperlinks (like here on lemmy, I’m clicking around, but my URL still shows slrpnk.net/... - yours probably shows sh.itjust.works/...), or if you try to do something like follow/reply/boost/etc on another instance, it’ll prompt you to connect from your own.

So like here I’m looking at a post on mastodon.social, which I don’t have an account on. If I just type in my home instance in the pop-up modal there, then it’ll complete the action from my home instance. If you’re already signed in on home instance with a cookie then it’ll to it automatically.