that the source code is open to be seen, cloned, or be redistributed. This also depends on the licenses – what users can do with the code – but generally you could redistribute your own flavor or instance of a specific social media.

the source code does not include user data like upvotes or comments that is (in the case of Lemmy) stored by the specific instance.

not too familiar how decentralized networks something like how a post cumulates upvotes on a specific server is something I’m not too familiar with. Point is, if you look in the Lemmy source code, it should not include user data or metrics. that is all stored within the users specific instance.