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.