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?

less thing to worries like you dont need an email to use it from single user instance, lemmy now dont have 2nd authentication like totp at the moment and it may have risk to get pawned and leak your email address so yeah it is better to run your own single user instance