Lemmy is unique on the fediverse, in that it has communities that you can subscribe to. The closest equivalent is community groups on Friendica. Pixelfed is more similar to Mastodon in that you can follow individual accounts, or hashtags/search terms, which is how you subscribe to different content in your feed. For example, I follow #catsofmastodon and see all posts tagged with that in my feed. Hashtagging your posts is very important if you want it to be seen. Lemmy is more oriented towards discussions, which is why communities make more sense in this context than on Pixelfed or Mastodon.