It's got a long, long way to become anything close to Twitter.
- I click "Follow."
- Servers talk to each other.
- I get redirected to a URL with a unique hash on the other server. It tells me what I'm agreeing to and I agree.
- I get redirected back to a URL with a unique hash on the original server.
There are a lot of other protocols possible. This one doesn't rely on CSRF or other features, so it should continue working even with security features of future browsers.
In that case, you can do it by selecting your home server. That's a search drop-down. Something like mastodon.social is there by default, and for historians.social, you might need to type in 'his' <click>.
I don't feel too bad about that either. It's like oauth, only instead of just having two options (Google and Facebook, typically), you have an unlimited options if you're willing to type a little bit more.
Or like SSO on many Microsoft web sites, where I type in my work domain or email, and it redirects through my work.