I have a 2023 E450 Cab. I love it.

I'm curious. What is the 12V battery used for? How is the 12V battery charged? What else is powered by the 12V system?

Used for most electronic functions on the car. Even pure electric vehicles uses a 12V battery. The 48V battery on the other hand is to assist with throttle before turbo spools up and power the ISG.
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As said above, the 12V battery is used for almost everything electric/electronic in the car. 48V is not safe near electronics, so there's a level of insulation. Most electronics use less than 12V, 5V being most common, so even the 12V has to be stepped down by smaller power supplies inside of the various modules. The 12V battery is mainly charged via a DC-to-DC converter which steps the 48V down to 12V. The 48V is used for system that need higher power such as the electronic motor when it does torque fill during gear shifts, electric steering, some active suspension and active roll stabilization and starting the engine. There is no longer a 12V power starter motor, instead the engine is cranked by the 48V motor. It is capable of spinning the engine up to idle rpm or higher and then the combustion sets in smoothly. This allows for a smoother start/stop system and a smooth transition for electric to combustion in general.

Something many people overlook is that the 12V is still the most critical battery even in EVs and continues to be referred to as the starter battery. Because if it's flat, you can't start the car, even if it's an EV. None of the electronics will boot up, so the car is dead if the 12V battery is flat. This has been an issue with EVs as the 12V battery often only gets charged while the car is on. The high voltage battery in an EV gets disconnected via power relays for safety, and those relays are controlled by the 12V system. Manufacturers have added fallback systems recently where if the 12V battery gets low, the high voltage battery is temporarily connected to charge up the 12V battery and then gets disconnected again once the 12V battery has sufficient charge. I don't believe they've added this to cars with 48V, so the 12V in those can still go flat and leave you stranded.

Electrical Engineering has a distinct field called power electronics. 48V and up falls under that, because under ideal conditions, 48V can kill a human being if you manage to get a current flow through your heart. That's why low voltage electrical engineers don't wanna be near power electronics. It's not safe.