Well partially correct, they are different servers and yes you can use any instance to view other communities but lemmy.ml/c/bob123 and lemmy.fmhy.ml/c/bob123 aren't the same community despite having the same name. Both would be accessible via either instance e.g on fmhy you could access lemmy.ml/c/bob123 from lemmy.fmhy.ml/c/bob123@lemmy.ml and on lemmy.ml you could access lemmy.fmhy.ml/c/bob123 from lemmy.ml/c/bob123@lemmy.fmhy.ml/
Like right now you're asking asklemmy from fmhy using the url https://lemmy.fmhy.ml/c/asklemmy@lemmy.ml but if i went and made a community called asklemmy on fmhy it wouldn't be the same community as the one on lemmy.ml
Hmm perhaps my explanation is too confusing ๐
Not that im aware of but what I have noticed is that you can cross-post across instances. Although most people just look for the most active community for the particular topic and follow it