Im joining in on the reddit ditching thing, and was kinda worried at first that i wouldnt be able to like use it the way i did reddit as it feels like a whole new place, but after engaging with posts and people and actually being a part of lemmy rather than being lurk mode all the time i was pleasantly surprised with how easy it is to become a member of the community, theres a reasonable amount of subs (or whatever the other word for em is) that fit my interests, enough linux content and shitposting for my liking, and the overall random posts made by people equally fed up with Leddit. (also i admit i used reddit a little cus there was this post on the fedora sub showing how to fix a sound issue i been having after a recent update)
I'm starting to understand how all of the individual Lemmy servers are connected and it's awesome. Understanding how Lemmy is fundamentally different than something like Reddit makes me very appreciative. I think something like Lemmy is the natural future as corporations continue to try to milk the wallets of the average person.
They have to subscribe to the community (it looks like a user to them, so it looks like @[email protected]). Then they can see the posts from that "user". From there, you see the posts and can reply to them just like they were any other user.
I don't know in a "what to click sense", but lemmy and mastodon differ from reddit and Twitter in that they're open source running on open standards. There's no proprietary walled garden to protect.
The underlying protocol is called activitypub. Think of lemmy and mastodon as different interfaces for viewing the same data.
If you just want the bare minimum, the ActivityPub article on wikipedia isn't bad. For a deeper dive if you want to get technical, w3.org has a much more detailed explanation.