"The post-security connector between Terminal 2 and Terminal 3 has opened at San Francisco International Airport.
You can now go between Terminal 2 (Alaska) to Terminal 3 (United) and International Terminal G (Star Alliance), all connected post-security."
Among other things, this allows AS passengers in T2, without having to reclear security, to access:
-AMEX Centurion Lounge in T3
-United flights in T3
-International G's Star Alliance gates
-Friends, family, coworkers in T3/T2 restaurants and lounges before or after flights
SFO states that another new airside connector from T2 to T1 will be complete in 2024, which would also allow post-security access to International A gates.