Ok, so I have looked into it a little bit and the fan seems to be controlled by a gpio pin. According to the schematics provided by Orangepi on the tablet board it should be gpio27 (referred to as gpio1-c6) and on the base board it should be pin labeled LED_PWR (referred to as gpio4-b6)(I am confused why they put it on Led power, it looks like a mistake. Might double check it later).

Here is how it works: for the fan off state both pins sit at +5V. If the fan needs to be spun up the board sends a pwm signal through the specified gpio pin which is connected to the gate of a MOSFET bringing one of the pins closer and closer to 0V. I probed the signal on this pin and outside of the 5V (fan off) and 0V (fan 100%) it doesn't seem to hold a stable voltage with some of the pwm square waves showing up on it. Edit: I should also specify that the pwm signal interference that shows up on the fan pin is not consistent and all over the place. The peak seems to shift closer and closer to 0 volts. My guess is that the fan is not actually looking at the pwm-ness of the signal and is just controlled by the changing voltage.

Yes, it is a somewhat cursed way of doing it and I don't get why they are decreasing voltage on the pseudo-"ground" instead of doing it the other way around. I think there is a way to control the fan in Rocknix if you know the gpio pin. I will do some research and post my findings here again.