where you mount your work surface in relation to the XY home in lower left corner of frame
For the vast majority of us, our stock is arbitrarily placed on the spoil board, and a G92 is executed to set the origin point for the cutting to a point relative to the stock. Homing the machine is almost always done to square the machine, not to establish particular coordinates. There are exceptions.
Have the SKR PRO V1.2 board, the USB does not seem to operate
You need to give specifics about what software you are trying to use for your USB communications. Having the wrong baud rate is the most common reason for USB communication problems. It needs to be 250000 for Marlin.
Is the Z axis touch plate controlled by the Marlin controller or do you need to add G-code to a design for it to function?
How the touch plate is handled varies between the Primo and the Lowrider. In both cases, Marlin will descend until the bit touches the metal plate, but the Primo uses homing while the Lowrider uses probing (since the Z axis homes up for squaring on the Lowrider). You have to decide whether to use the top of your stock as a reference, or the spoil board and place the metal plate accordingly (and set your up CAM accordingly). Personally, I tend to use the spoil board as reference for contour cutting, but the top of the stock for other kinds of cutting like V carving and pocketing.
There is a built-in homing menu item in the V1 custom menu in Marlin mode as well as a homing control in the TFT firmware. But there is a significant problem using built-in menus for homing… they don’t account for the thickness of the touch plate. Personally, I use the following g-code on my Primo:
G28 Z ; Home Z
G92 Z0.5 ; Account for touch plate thickness
G0 Z5 F480 ; Lift the bit off the touch plate
I have it attached to a custom button in Repetier-Host, in a file on the SD card, and attached to a button on my custom pendant, so I have three different places I can use to run my Z homing code.
FYI: If you are comfortable compiling the firmware for your board, it is easy to change the V1 custom menu to take the touch plate thickness into account.