Originally Posted by
mrbofus
At SFO, after customs and baggage claim, you can go left to go to connecting flights, or go right to exit the secure area.
I just went through SFO a month ago and you do not remain in the secure area after clearing customs. Like all other US airports that I've been through, you have to recheck any checked luggage and undergo TSA screening once again before being admitted back airside.
As for OP's initial question about lounge access, even if you could remain airside without a connection, you wouldn't have access to most lounges other than those designated as arrivals lounges, unless lounge access was granted something other than F/J/status (e.g. paid membership).