Lemmy is a few years old, but was a slow moving personal project up until a month ago, so there are still some kinks being worked out. For finding communities, try using lemmyverse.net, I’ve found it has basically every community catalogued. If the community doesn’t come up in your search, then your instance probably hasn’t federated yet and just wait a bit, when you search a community for the first time your instance will start loading it in and it will show up in a few minutes.