why is there a 12 volt battery in the hybrid

[Groo]

There are federal laws that regulate manufactures signals & lighting to be 12V, they also require low voltage (under 50V) to user controls to reduce shock hazards
Manufactures have been trying for years to get the laws changed to 48V

I know the Dodge GrandCaravan uses 5v lighting, the natural voltage for LEDs, that way each bulb doesn't need to be lowered
I believe safety BS kicks in at 60volts.
42volts is used for a bunch of automotive stuff. I thought that was the standard selected for the "next step" by the industry as a whole.
I know a bunch of mobile equipment sensors are rated to run from 9volts to 36volts or a regulated 5volts

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[HeyBales]

Dad was on his second Prius, years before I got my Maverick. My understanding is that Ford and Toyota worked together to create the eCVT, and the two systems are very similar.
On the first Prius, it would chew through 12v Batteries because the HVB cooler would run after shutting down the system. Then it would no longer start because the 12v battery is what ran all the computers. The first Prius had a crap alternator that wasn't strong enough to keep the 12v battery charged with Dad's short trips. The second one does not seem to have the same problem, but he moved further out. The Maverick looks to use a DC-DC converter to charge the 12V battery, so I expect things to be better. I hope so. My commute is even shorter than Dad's was.

Don't expect too much!

Yes - DCDC Conv to power the 12V system after turned On, and charges the 12V battery.

The Battery Management System (BMS) can have such fine control now of exactly what current Amps and Voltage to supply to the 12VB - it's basically a battery charger.
But you know how your battery charger needs maybe 2-4 hrs to top off a battery safely - well that don't work while driving with much shorter trips.

So the truck strategy for AGM, likely for safety aspect of overcharging a battery in the cabin - is float charge up to 92% State Of Charge (after initial under 10 min higher but decreasing current charge), then allow the 12VB to power the 12V system and discharge down to 87% SOC, then float charge repeat.
Float charge for a 5% increase can take awhile driving.
If not driven enough - you don't get there.
So now battery maintains at lower voltage, and slowly starts to sulfate - degrading more.
At least degrading the AGM battery takes longer - it's tougher.
Also, if in that discharge range, BMS will go into full charge if headlights/parking lights on, fan over spd 5, or truck thinks a trailer is plugged in via the hitch cable used.

Oh - Ford & Toyota didn't work together on the eCVT. They are similar for a different reason.