You program/configure your keymap into the keyboard’s firmware so it is independent of your operating system. If you use Mac, you put your Mac specific keys into the map where you want them. It’s totally up to you - if you want every key to be “a” you can do that :)
I think https://github.com/KMKfw/kmk_firmware might be what you are looking for: It exposes itself as flash drive in bootloader-mode and the files on there are just regular Python scripts. If you search for “KMK macro pad”, you’ll find a bunch of projects - most of them diy. If you want to start from scratch, Jan Lunge created an awesome tool for setup and keymap adjustments (video https://www.youtube.com/watch?v=RtYJYFMWjNM, repo https://github.com/JanLunge/pog) - with the idea being that after setup you do not need the tool anymore to adjust the keymap.