I have a new Lemmy server (lemmy.todayyoutomorrow.me) and I've noticed only communities I subscribe to show up.
The idea was to have my own local instance but I don't see how I can find new communities without using another instance first and finding those communities there and then manually adding them to mine. I have found the following two github projects:
Use lemmyverse.net to find communities across all instances. It will make you search a lot easier, and show you when a community exists on multiple instances