If you host your own, do you need to establish federation with all other instances or only with the ones you want to use communities from?

If I only federate with lemmy.world, would I be able to see comments on /c/selfhosted@lemmy.world on my instance made by a user from lemmy.ml?

Would a user that reads /c/selfhosted@lemmy.world on lemmy.ml see my comments, if I only federate with lemmy.world?