You can only support, see fixtures, and purchase players from Managers on your server. The players must be three to five star from your perspective for you to be able to make an offer.

You can view all your friends teams, play friendlies with them and send gifts no matter which server.

You can see matches from your Cup, League, and Champs League even if they aren't your friends.

I have no clue what you mean by sponsorship friends, if they are invitee they will be on same server.