large jets use the ram air turbine for power. It drops from the belly of the plane and the airflow turns the turbine.
Batteries are heavy and a plane demands a lot of power. Yes there are batteries but not enough to run all systems.
I checked the 737 doesn’t have a RAT as standard equipment at all, it’s a custom order so that could explain the lack of RAT deployment clearly in the videos there is no turbine hanging down from the fuselage. Boeing expects the APU to be running should both engines fail or be unavailable. The APU is the backup third redundant system in the 737 series. So if the FO killed the good engine by mistake and didn’t have the APU already running they were in deep kimchi at that point.
This is a 800 series it has three hydraulic circuits A,B and stand by <<< this is the APU driven one. the manual says that either engine’s hydraulic pump can still be driven by ram air via windmilling at speed. It doesn’t specify what the minimum airspeed is for windmill operation it looks like under 200 knots is below the minimum windmill speed. Without that APU they would’ve been running on manual reversion controls without hydraulics at all. Plus too slow for a windmill restart on the good engine ,since no APU means no high pressure bleed air either. Boeing says there are two electrical driven pumps one each for A&B systems with batteries for 60 min of basic instruments and also basic hydraulics this was their justification for not being required to have a RAT by the FAA. Someone would have forgotten to link the battery bus to one of the electrical driven pumps on the hydraulic circuit of the good engine. Those batteries also supply DC voltage to the APU starting motor/generator bus so they could have spun up the APU and then had full electrical and hydraulics plus HP starting air. Someone messed up and messed up badly this aircraft has triple redundant back ups plus manual reversion flight controls and a full 60 min battery for flight instruments no way those were depleted in 4 min driving the hydraulics ou the A or B system. The FO didn’t tie the emergency DC bus to either the hydraulics nor the APU nor the bus with the black bdoes which would have been primary flight instrumentation bus as well.
No RAT on a 737.
Certainly there could be one to run the black box??