Welcome to FlyerTalk,
jxcov!
So you are doing an immediate turnaround SFO-FRA-SFO! UA flights from SFO usually arrive at Z20 gate and provided you have a BP for the return flight you can just stay in the departure area without going through security or passport control. There's very little seating around gate Z20 but you'll find restaurants and cafes if you walk towards Z50.
P.S. No, we don't
all know United is retiring their 747's this year