I’ll heading down to L.A. from NorCal. Does one need a security key code to enter the lobby for bathrooms?

Yes, a key code is required. Tap the station's icon on your navigation screen and it should show you the code. The owner's lounge and 40 charging stations make this one of the best locations in the west.

I think your Tesla app will have it under 'charging, nearby superchargers'?

yes, you need it to enter the lobby and bathrooms