On: “I would buy a Mac today if I was not working at M...

[Fnoord]

I got that working with Karabiner Elements but how do people get this to work on Windows and Linux?

[rgoulter → Fnoord]

KMonad supports customisations like that. It's cross platform (macOS, Linux, Windows).

Though I use a fancy keyboard with QMK firmware, which allows customising this without needing to change settings on the computer.

[stavros → Fnoord]

I do it with xmodmap on Linux.