Note
This is my summary version of https://gist.github.com/sonicprod/f5a7bb10fb9ed1cc5124766831e120c4 specific to my use case. Credit (and a big thanks) to Sonicprod (+ others) for their hard work on the original!
How to make a dedicated MAME Appliance on a Raspberry Pi 4/Pi 400.
https://www.raspberrypi.com/software/operating-systems/#raspberry-pi-os-64-bit
Download Raspberry Pi OS and write the image to an SD Card.