How to make sdc/usb bootable drives

I have been struggling with this for a few days and still am. When I downloaded Ubuntu Mate 18 and installed it on an sdc card using etcher I got two file systems: /root and /boot. I could locate files like /boot /usercfg.txt and edit it. Howerver when I download Ubuntu Mate 19.10 and installed it on an SDC card using etcher I get: "Ubuntu-MATE 19.10 amd64". No /boot or /root/ . Now if I want to take this SDC card or thumb drive with "Ubuntu-MATE 19.10 amd64" and install it on my laptop I can hit F12 during boot and tell the laptop to boot from the thumb drive. Ubuntu Mate 19 will then "allow me to take a tour" or install it. Now for the question. On a Raspberry Pi 4 I do not think that pressing F12 during boot/startup will allow the option to boot from usb. And so far I cannot get a Pi 4 to boot with Ubuntu Mate 19.10. I know about the usb "problem" but cannot locate the ussercfg.txt file after installing with etcher. Any ideas/recommendations. i

I seems that the new installation method is to boot from a thumb drive and interrupt the boot process so that you can access the Ubuntu 'live' installation by hitting F2 or F12, picking your usb device and then taking a tour or installing. If I am correct how will this work with a Pi since they do not have a bios.

Thank you. Any and all suggestions welcome.

The architecture (ARM64) of a raspberry pi is different than x86_64 (Intel/AMD 64 bit).
You cannot use the "Ubuntu-MATE 19.10 amd64" iso on a pi (different arch).

So I picked the incorrect download. Brilliant!!! Any idea of when Mate for the Pi 4 will be ready?

I want to revive this topic with a slightly different angle; SD bootable on x86_64. I've been trying to make a bootable SD card for awhile, since for some people it may be their only reasonable option (think old netbooks with only a single USB slot) and I cannot seem to crack it; GRUB keeps on complaining about the kernel not loading and I've failed to make any SD media function post-install.

Any tips?

@tiox, maybe this article helps ...

This advice isn't helping my case; The instructions are a tad bit too confusing, and don't present steps to verify the chroot command actually does anything. When i run initramfs-tools nothing happens despite following these instructions, with my hardware and fstab configuration to the best of my ability.

Are there any instructions like for what is presented in that URL which don't suck?