Arnold’s Drive-In was originally “Arthur’s Restaurant” at 1001 S. Victory Blvd., Burbank, CA, in the pilot and first (few?) episodes.
Really though, the back-lot set was at in a corner of the Paramount Studios lot. In several moments when filming at “Arnold’s (Al’s) Drive-In”, you can clearly see the buildings next to / behind the lot. Those buildings are still there today. You can make out the curved arch of the breezeway of the apartment building (Hollywood Dream Suites Apartments), and the attic vent (now behind a tall palm tree) of the house to the right of the apartment building.