Lemmy is also built on ActivityPub as part of the Fediverse and is arranged into topics (called communities). E.g. you can follow retrocomputing@lemmy.sdf.org (or web interface https://lemmy.sdf.org/c/retrocomputing) and only see retrocomputing posts.
Since it's all ActivityPub, you can follow Lemmy communities from a Mastodon client but the UX in that case is pretty bad (e.g. Mastodon servers won't backfill posts so you won't see any history).
Since it's all ActivityPub, you can follow Lemmy communities from a Mastodon client but the UX in that case is pretty bad (e.g. Mastodon servers won't backfill posts so you won't see any history).