How does federation on lemmy work?

I cannot see posts from lemmy.ml. I only see a few but not a lot. When I go to https://lemmy.ca/community/2573, for example I don't see any posts however when I go to https://lemmy.ml/c/embedded_prog, while not logged in because I don't have an account on lemmy.ml, I see posts. Is it just me or is it an effect of federation?