Various upgrades modes that you can enter

I’m looking at Boot Into Upgrade Mode | Khadas Documentation and the page describes how to enter various modes (upgrade, maskmode, uboot, serial) but doesn’t mention how these modes work and what you can do in these modes at all. Where can I find this information? When I try to do this, the blue led flashes thrice and the device just reboots.

If you just want to flash the image to eMMC with USB-C cable, you can choose TST to enter upgrade mode. Please make sure to connect the board to you PC with the USB-C cable.