the red chassis fan header is only designed for the oem front fan with a specific ohm and power rating.

if you plug a non-compatible fan, it would not spin at the expected rpm.

bios controls the fan implicitly and user has no option to adjust fan speed.

I have searched for an official system fan on the internet and Dell website for the 7090sff. They don't apparently sell them or even mention them. They only talk about the CPU fan. I think that I will just cut the pwm wire or buy a 3 pin to 4 pin cable. This should make the fan run at full speed.

fresh out of box 7090 SFF video shows this fan mentioned in manual is a ghost and was not installed in Dell factory in practice.   

DELL 7090 SFF UPGRADE RAM & NVME SSD

When I start the pc, the pwm front fan runs at high speed for a moment, then runs very slowly, but it does run. So it does have power but the control is unknown. I have an i7-10700 using win 10 pro.

@Leexxx​ 

I have install a 3pin to 4pin adapter cable and the ffan runs full speed.

good.  the red front SYS fan header is 4 pin.  OP tested with a 3 pin fan using 3 to 4 pin adapter and fan is full speed.