Calibrate Quarq power meter on Fenix 6 or Edge 530 or zero in SRAM app?

All power meters are different, so it is best to follow the manufacturer advice regarding calibration. Some are better at dealing with thermal change, for example. 

FWIW I have Assioma Duo pedals, which should not need recalibrating day to day, but pretty much only when refitting to a bike. Nonetheless, I will usually calibrate on every use, just to be sure. It literally only takes a few seconds. 

As for which device to use, I'm not sure that it matters, but I guess your primary recording device would be the most logical option if there was any doubt at all.

Note that my experience is limited to the Assioma pedals and my Tacx Neo trainers. I'm definitely not a power meter guru. :-) 

No, You dont need it (only before indoor ride) if the magiczero (autozero) is enabled in the Quarq (firmware #10), with this feature the Quarq is always re-zeroing itself when You are coasting (this is why better to do before indoor ride due to there are usually no coastings or if yes, sometimes not enough "clear" to do this autozeroing flawlessly). But if You want, You can do manual zero calibration before every outdoor ride also and in this case from start till the 1st coasting the measurements will be (probably) perfect. (but more accurate for sure than without it)

Doesnt matter with what kind of device You do this manual calibration, it is only a command message and processed in the spider so choose the easiest method (so only 1), if You set the "calibration prompt" enabled in the 530 then everytime when the Quarq is waking up You will get a pop-up message in 530 for calibration and go on.