Japan is an oddball by being 100V.

US is 120 and that extends pretty far south (and north).

https://en.wikipedia.org/wiki/Mains_electricity_by_country

I’m semi amazed motor vehicles are as standard as they are with 12V and the same socket worldwide. I guess the tobacco industry is a great unifier.

I find the standard voltages pretty interesting. The 230 V standard, for example, is mostly a lie. In reality, Britain and former British colonies tend to run on 240 V, and continental Europe/Asia/Africa tends to run on 220 V. The 230 V standard includes wide enough tolerances so that no one needed to actually change anything. I've never actually seen 230 V, the supposed standard, in real life.

I've just measured the voltage in a socket my home (Germany) and the multimeter says 231 V. (And it's nighttime, so no solar generation from houses in the neighbourhood potentially distorting the local grid.)

NATO compliant vehicles are 24 volt as far as I know

How, exactly? The airlines have absolutely no way to know what shoddy electrical device you bought god-knows-where you're plugging into mains power in their airtight travel-coffin, packed with hundreds of people, hurtling across some ocean.

> Almost all the international flights I've flown have had power outlets

Seems deeply unusual to me, but I won't dispute your experiences. I've flown internationally fairly often, and in my experience power outlets are rather uncommon (at least in the eastern hemisphere, flights to/from the Americas may differ, I haven't flown around there for many years).