I just use the communities tab [0], that's on every instance. Then select All to get a list of all communities federated with this instance sorted by size.

Other wise I just, use the search bar. I just tested it with the goal of finding a photography related community. Found it with the first search result from "photography". [1]

Another example, I wanted to find a risc-v related community, one simple search [2], boom, I found one [3]

[0] e.g.: https://feddit.de/communities/listing_type/All/page/1

[1] https://beehaw.org/search/q/photography/type/All/sort/TopAll...

[2] https://beehaw.org/search/q/riscv/type/All/sort/TopAll/listi...

[3] https://lemmy.ml/c/riscv

Edit: Also, idk how I missed it, but the communities tab [0] also has a search, so there you go, I don't even know what else you'd want.