Few Question re upgrading Head Unit

[StepOnIt]

Hi I am thinking of upgrading the current head unit on my VIOS 2018 1.5G (Head Unit model PX-3834A-I), but need some info on the following:

The original head unit is currently supporting 4 cameras on the car, font, back and 2 on the side mirror, so when parking/reverse, the head unit is displaying a split screen, on the left is the front/rear(depends on forward or backward at the moment) view, on the right is the bird eye view of the car.

- My question is, is this split screen image pre-combined b4 feeding into the head unit as 1 single source by some other module, or feed as separate sources into the head unit and handle by the head unit? If I install an aftermarket HU, will I lost this split screen functionality? 

- When I look at the HU options on the market, some offer 1 camera-input and some expensive ones offer 2 cameras input. Are those 2 cam input types required for the split screen function, or do I misunderstood?