If you click on Communities and select All, it allows you to search for Communities across all of Lemmy-Land, not just the instance that you’ve signed up on. I think that every instance can have its own separate community with the same name, but in a lot of cases that may even be desirable.

As far as moving users/communities between servers, that’s Nomadic Identity. As far as I know, the only Fediverse project that implements it is the Zot protocol that Streams and Hubzilla are built on. ActivityPub does not support and it’s not on the roadmap as far as I know.

I don’t know what the rules are on lemmy.ml, but some instances let anybody create a new community. There’s no guide or doc that I know of, but it’s as easy as making a post is. I just looked for a calculator community and couldn’t find one, so I made it. If lemmy.ml won’t let you make a community, either ask the mods there to approve one, or sign up on an instance that isn’t as locked down. You can even make your own instance and still federate with the rest of the Lemmy-sphere.

If you want to see a specific community that doesn’t exist, then set it up and post some content! If you want the type of places to hang out that you had on Reddit, you need to create it, nurture it, and grow it. There’s no guarantee that Lemmy will be the place to land during the Reddit exodus, but it is a place to land, and a damn fine one at that. Putting my money where my mouth is, if you ever want to talk about calculators head on over to !calculators@midwest.social