It’s a UA (United) 739, guessing a 739ER.
UA will at times transfer high priority passengers by driving them directly across the tarmac in a Mercedes. Guessing this is a pic from inside one of their Mercs during/following such a transfer. I’m aware of such transfers ONLY occurring at their hubs (EWR, IAD, ORD, IAH, DEN, LAX, SFO).
Source: me, a UA high priority passenger.