Your handle does work for all of the various Lemmy servers. But to access them it’s like your email, you wouldn’t log in to your Gmail account from Yahoo. Yahoo has no idea what your Gmail username and password is. So how can it let you in? And like email because both servers speak the same protocol you can interact with other users on other servers just like if you had their email address.
In your case lemmy.one is your email server so to speak. You can access any other Lemmy community or set of communities on another lemmy server by searching directly for their address on your home server or if someone else has interacted with another server already that server’s communities will show up in your home server’s All list and you can see those posts there and interact with them as if they were local to your home server.
I play AD&D (currently playing though Temple of Elemental Evil). Why? Because the people I play with are proper OG nerds, they played D&D when it was originally released (my DM still has his original red box). As a young whippersnapper coming from 5e it’s different I’ll give you that. It’s a lot harder to min/max and I appreciate that. Mostly it’s just fun seeing how DnD evolved, the fact that shenanigans will always ensue means it’s still fun as ever but just in it’s own special way.