Originally Posted by IAH-OIL-TRASH

You'll be checked in for ANA flight when you check in for UA flights. You can pick up boarding pass for ANA flight at Narita ANA desk past transit security.

Originally Posted by 747FC

Can't speak for NH specifically, but whenever I fly partner airlines as part of my UA trip, I first check in with UA. If they do not provide the partner boarding pass, I then log onto the partner app or site and get it from them. Also, if you have lounge privileges, you could just stop into an NH lounge or Star Alliance lounge and pick one up.