Originally Posted by eweow3

Ah, my apologies, just noticed the UA in the thread title.
Here's the deal for a fully airside transit:
No checked bags: This is key. If you have checked bags, you'll have to go through the immigration to collect them and re-check with Starlux. Don't expect any assistance from UA staff at SFO for your SGN leg, as it's operated by Starlux.
Your best bet is to do OLCI for the SGN flight and hopefully retrieve a mobile BP. If successful, you're good to proceed directly to the gate. They might do a quick document check at the gate for the SGN leg, but that's it.
If you can't OLCI or can't get a mobile BP, head to the Starlux transfer desk and see if they can assist.
Worst case scenario (and unlikely, IMO): If starlux can't issue a boarding pass and help you check in at the transfer desk, you'll need to clear immigration and check in landside. But I really doubt it'll come to that.
Btw, as things stand now, UA lands in T2, and Starlux SGN flight also departs from T2.
Hope this helps!