mastodon is essentially ‘twitter’. also called ‘microblogging’. people post things that other people can see because they subscribe to ‘people’
the hashtags are supposed to be used to aggregate posts between all those different people to help find common posts.
lemmy does none of it, as it is not capable of ‘microblogging’
there are a few platforms that can do both the ‘forum based’ stuff (threadiverse) and the ‘twitter based stuff’ (microblog) like mbin
on my instance for example, you can follow people on mastodon and interact with all the lemmy content
the follow-up is also a great read
https://dustycloud.org/blog/re-re-bluesky-decentralization/
tldr;
bluesky is neither federating nor decentralized, but is a great twitter replacement (not that theres anything wrong with that)
e. also, the longer it exists, the less incentive they have for ever federating or decentralizing. so it will for, all intents and purposes, never do those things regardless of what the board that runs it professes.