Can MacBook Air (2020) be charged from airplane USB?
I read somewhere that the new MacBook Air computers (the ones with USB-C) can be charged slowly from any USB source using a USB-C to USB-A adapter.

Yes.  This is correct.  The Type C ports on your Mac conform to the USB-PD (Power Delivery Spec) meaning it will negotiate for the maximum charge.
The default charge that comes from a USB-PD device starts out at 5V - this, coincidentally is the same voltage that legacy USB chargers output.  If it can negotiate up (to a max of 20V) it will do so.  Otherwise, it will simply draw the maximum which is no more than 12W (5V at 2.4A)
So, if you use a 5W charger (airplane, car, etc.), it will take forever to charge that MacBook (you'll drain battery if you attempt to use it while charging), but it will charge.  It will go faster if it's a 12W or 15W port, but still be quite slow.  As for the power draw, it will draw what it can - if the airplane only supplies 5W; that's all you'll get.
Your best bet is to get a proper USB-C travel charger and hopefully the airplane will have the 12V sockets (car cigarette lighter port) rather than USB ports.
As for the cables you need, if it's a USB-A port, just the USB-C to A adapter you already referenced and a USB-C cable which you should already have.