G400D dev board touch screen schematics

I looked at G400D dev board schematics, cannot find any pins related to touch screen. Where are they?

do you see Pin 37 to Pin 40 at 4.3" Display near G400-D SOM ?
starting with T.

What does T.XR T.YD T.XL T.YU mean? Does it use I2C to communicate?

I2C would use 2 lines (except ground)
those pins are going directly into the MPU (the 10-bit ADC supports 4-wire and 5-wire resistive touch panels)

What LCD panel are you using? Is it capacitive or resistive touch?

For resistive and a more reliable touch I would recommend you use an external touch IC for 4-wire panels. You can then use an interrupt to handle the touch and simply pass the touch back to the low level drivers. The ADC one I believe is polled and if you have a busy design touch is very sluggish. Microchip and TI do compatible devices for this. SPI interface.

For capacitive, just use I2C.