lemmy is like reddit. mastodon is like twitter. the difference between each and their corporate versions is that lemmy and mastadon each are services that don't run on one, central server but on many, independent "instances" (servers) which "federate" with each other and with the larger "fediverse" where they're able to interoperate.
Lemmy and Mastadon, being different but similar services, can interoperate a little bit, but only so far as that lemmy content can be seen in the Mastadon app (if configured properly) but not vice-versa.
does that answer your question?