You can just search for something like Android and it’ll show all the communities including the instance it’s on and join; you don’t need multiple logins.
The app I use, wefwef, also can view all communities in the feed along with your subscribed or just local instance communities. I’d assume liftoff is similar but I’ve never used it.
Exactly what gatob said. Lemmy is the platform that all instances (Lemmy.world, lemmy.ml, etc) connect to.
All apps work with all instances. There’s jerboa, connect, wefwef (soon renamed to voyager thank god), liftoff, with more coming soon (boost & sync of reddit fame)