What is necessary to boot Linux on a Mac?

I've seen some various guides online about different ways to boot Ubuntu from an SD card or a USB stick. They have some variation with regards to making it always boot from the external memory or fully installing it on the MacBook, for example. Apple does not seem to provide any official information on how to do this. Ubuntu provides information about downloading Ubuntu to an SD card, but not what it takes on the Mac end.

I'm curious to understand underlying principles. What is necessary to get the Mac to boot Ubuntu, if it is on a USB stick or SD card?

Does the Ubuntu download need to be formatted a certain way, with diskutils? Why? Does the Mac drive need to be partitioned a certain way? Why? Does anything need to be done with the booting software, grubloader?

At minimum, is it possible to just download Ubuntu to an SD card, stick it in the computer and turn it on? Why would the computer immediately know to boot from the external memory?

Solution 1:

There are several tutorials online that tell you how to format a bootable drive and use it to boot up. They usually instruct the user to download an image, use something like balena Etcher to "flash" it, and then select it as the boot drive during the startup process.

https://www.ecosia.org/search?q=booting+ubuntu+from+usb+mac
This should help you.