All good, same boat but from what I can tell it's just a different approach to how most people have grown accustomed to the web.

You can technically follow communities and users from Lemmy on Mastodon, though the two platforms have different UIs. I was recommended to register for two different instances as they let you interact with each one to the fullest/intended way. However you can still communicate between the two which is the fediverses' biggest strength imo.

E.g. I found this comment via lemmy. I copied the link and pasted it into my mastodon search bar. Once found, I can comment it through my Mastodon instance. The difference is that I cannot do unique lemmy interactions such as make lemmy-posts or downvote because those aren't shared functions on ActivityHub between Lemmy and Mastodon.

So, in terms of interaction, use a lemmy instance (though try to register for a less populated one that fits you're identity to distribute the traffic/server load). Registering may take a bit since it's run by actual people.

In terms of communication, you can use either instance. The key is using links and the search bar. I cannot overstate how powerful the search function is in the fediverse, if you have the right link, you can find any post in the fediverse.

Do you mean at the technical level or UI level?

At the UI level, just have a look at https://kbin.social and you'll see a lot of very familiar posts. Also their posts show up here, just like any normal post.

Mastodon is a bit trickier. If you put hashtags in your post text, I think it shows up like a "toot" for users following it? Sorry, not a Mastodon user, don't really know how it works.

At a technical level, Lemmy is built on the ActivityPub protocol. It's how Lemmy servers talk to eachother, it's how Mastodon servers talk to eachother, and it's how a hell of a lot more services talk. Best analogy I've heard is ActivityPub is like the email protocol for social media platforms.