Not sure how I got to lemmy.one but when it said I had to login to comment on something, my userid/pwd that works here didn’t work and the registration link said user registration is closed. Aren’t lemmy logins supposed to work across the whole federation? Or are there multiple federations and lemmy.one is entirely different? Maybe I misunderstand the whole scheme of lemmy.
The logins aren’t federated, the content is. Each instance receives a copy of everything, and normally you browse other instance’s content from your home instance. In your case you’d access lemmy.one’s content from your home instance, lemmy.world. If the logins were federated we wouldn’t need those domains after our usernames!
The email analogy still works for this: if you’re on Gmail, you don’t go log in to Outlook to send an email to your friend: from gmail directly, you send an email to your friend and Gmail’s server takes care of sending it out to Outlook.
There’s browser extensions to help go back to your home instance, as the linking on Lemmy is sometimes a bit weird and you do end up on other instances every now and then.