if i delete a post from my mag that is from a user from lemmy, will it be removed on both ends (both kbin and lemmy)?

It depends on what the remote server does with the federated delete command. You should generally assume that anything you post to the fediverse is permanent.

This is only from what I have seen and I have no knowledge of how federation works:

No. Ex: there have been many spam posts on food@kbin.social that I made a post about. I found out from that post that they were removed from the kbin mod, but I could still see them here on Midwest.social.

See this post for more info. There is also a link in there about this known issue.