You need a checkpoint with the right model of ID checking machine, AND an agent that is trained in how to use that type of machine. At SFO that's officially only at the Terminal 3 F1 checkpoint, although I have seen what appears to be the same machine at the Intl G Pre checkpoint line too - but it's never been staffed/in use when I've been there.

Apparently you can now use the CA mDL at any checkpoint with those machines anywhere in the US. There's a list on the TSA website at https://www.tsa.gov/digital-id (the map appears to not be accurate - at a minimum it doesn't show SJC despite that being listed in the table below, and definitely having the machine at the Terminal B checkpoint).