Mastodon and Lemmy are both built on top of the same protocol: ActivityPub. That said, they provide a substantially different user interface. Mastodon is focused on microblogging (basically, a Twitter clone), where you follow individual users and boost (retweet) or like posts. Lemmy is focused on communities (basically, a Reddit clone), where instead of subscribing to users, you subscribe to communities. There is some interoperability, but it kind of like a Spanish person trying to speak to a Portuguese person at this point.

Long story short, even though the goal is for all content to be accessible regardless of which platform you are using, there is a lot of content that can only be understood if you are using the correct one.