Help needed to understand LCD screen management (RGB data registers)

Hi, I am a fresher using STM32H7743I-EVAL2 board.

We bought this demo board to test the performances of the LCD screen and STM32 in order to develop our final product.

MCU demo boardLCD demo board

This screen has one 8-bit R register, one 8-bit G register, one 8-bit B register (RGB) but we can see only 6 bits used.

1st question, why 2 bits of each RGB registers are not connected?

Also, we have subcontracted a firm which has designed a prototype board according to our needs, with a smaller screen.

LCD Prototype boardWa can see the R and B registers only use use 5 bits, then I think we will need to modify the source code of the evaluation board to fit with the smaller screen of our prototype.

I think there is a library or driver in the code of the evaluation board to manage the screen?

Could you help me in finding that code and help me in understanding what changes make to work with our smaller screen?

Thank you very much for teh help.