Below is my private response to @OntheMain:
I don’t mind helping if I can, but let’s not spam your original thread that was mainly about Coreboot, if I understood correctly. Feel free to create a new public thread about e.g. USB booting and add this reply there if you like. Updating the BIOS is something entirely different from updating your operating system and the software running on it. You can save updating the BIOS for later.
First of all, I have hardly touched a Mac in my life, and I don’t think I ever will, so I know nothing about Mac specific stuff. What I can say is this. A regular USB-sticks holding a couple of GB should suffice for booting a Linux distro and is very cheap to buy. In order to install a Linux distro, e.g. on your Librem 15(?), you first need to download the .iso file in question and save it to the hard disk of the machine you used to download the file - not to a USB stick. Then you need to use a special program (with or without a GUI) to create a bootable USB stick out of the downloaded .iso file. I don’t know what programs are available for Macs, but since the Mac kernel is Unix based the “dd” command may work from the command line. I’m not sure why you use the word “rebootable”. You just need a bootable USB stick, not really a “rebootable” one. Once you have created the stick, you put it in the machine you want to put the new OS on. Then, instead of booting from the hard disk of that machine, you let it boot from the USB stick. This is done by calling the boot manager by pressing F2, or something like that, right after pressing the on-button and starting up the machine. After this, a friendly distro like Mint will just let you follow simple instructions on the screen. I’m not sure how Qubes does it, but I really think you should try Mint first in order to get accustomed to this new world of freedom, but also responsibility. Please let me know if this explanations makes sense to you. Just keep in mind that I hardly know anything about the Mac world, except that I want to avoid it. 