The red one is simply a power LED with no monkey business.
The green one is connected to a GPIO pin. U-Boot never seems to mess with it. The schematic calls it LED10 and shows it driven by a labeled "GPIO0_B3. The schematic inconsistently (and in error) labels this as GPIO0_B5 in some places.
So we have a reset button (the button closest to the ethernet connector) and another button called a "recovery" button (or "upgrade" button).
The schematic shows the reset button connected to "VPP_OTP" on page 6 along with what is labeled as an overtemperature protection circuit. This goes to pin 6 on the PMIC, which is labeled OTP/Reset.
The schematic shows the recovery button (SW2107) connected to "ADCKEY" which is routed to ADC_IN1 on the RK3399. The schematic also misleadingly labels this as "RESET". An interesting use for an ADC input, if I do say so.
The board user manual discusses using the recovery button in conjunction with a USB-C cable to burn an Android image to eMMC. The instructions say to press and hold the upgrade button, then press the reset button and the board will enter "loader mode". There will be messages on the serial port, including one saying "rockusb key pressed". So they say (and I believe them), but I have never tried this. There is linux software called "upgrade tool" that can talk to this USB loader.
The user manual also discusses using "maskrom" mode to program the eMMC. This involves using tweezers to connect two points they show on a photograph in the user manual. This suggests that the process described in the paragraph above is not using the "maskrom", but rather a first stage bootloader that exists on eMMC.
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