I fly all the time for work and I use a portable charger with a USB-C to USB A cable and it keeps the switch at 100% while playing zelda.
I've never had a problem...
Airplane mode cuts power consumption. What brightness are you playing at? Volume level?
USB-A to USB-C does the trick, it even charges, albeit slowly. A long plane trip will trickle charge the Switch just like how a powerbank does for phones. You need USB-C to USB-C with PD capability to fast-charge while playing Zelda at full volume on headphones and max
brightness.
Any activity other than pushing the Switch hard in intensive activities, like if it's just idling on the dashboard, will top up the battery faster. I've seen Zelda topping up at 18W power draw (15V 1.2A) when measured through my USB-C power meter. That's in certain grassy area, in the rain, with Link's constant health warning beeping in the background, at max volume on headphones and brightness.
This is a typical average readout while playing Zelda (max brightness, max volume, WiFi on), charging using my USB-PD capable powerbank. Between 15V 1A and 15V 1.2A. That charging rate allows 1% Switch battery topup every 2 minutes.
Using USB-A to USB-C on the USB-A port on the powerbank gives a stock 5V 1.5A, and using the same controlled conditions (max brightness, max volume, WiFi on), I barely moved 2% battery charge into the Switch after 22 minutes of charging.
Yeah, the Switch doesn't take full advantage (or any advantage?) of the power delivery stuff on USB C. Been a while since I read that thread, but I think it doesn't pull more than around 2a regardless so as long as you're plugged into something that can put 2a through a USB A to C cable you're good to go in terms of charging while playing.
Only thing to be careful of with A to C cables is to make sure you get a compliant one as bad cables can botch the power negotiation and fry things. Seems to be less of an issue with C to C cables (maybe even a non-issue).
It's the reverse. USB-A to USB-C falls back to the lowest safe charging rate, 5V 1.5A, IRREGARDLESS of any possible fast-charge/quick-charge protocol it has. Even if the USB-A port advertises itself as a quick-charge port with 2A, 2.4A or even maximum 3A.
USB-C to USB-C however can the capability of carrying up to 20V 5A, that's 100W. Failing charging protocol negotiation between the charging device and the device it is charging, means that there's overcurrent draw potential to fry your device. Hence the caution for USB-C to USB-C.
The rest, what FX-GMC said.
USB-PD/C devices will provide 15v/1.5a to the Switch. Anything non-PD will provide 5v/1.5a