Personally, I use an external site to search all instances. The search on an instance, and the app, only search for communities within that specific/specified instance. I don't wanna have to muck about adding instances to Liftoff that I've no interest in making an account for, and then making the same search several times on different instances to find what I'm after, so I do hope they add a function to search the entire federation as well as the individual sites.