3 Questions .... probably not exclusively api but for my pi

I have recently obtained my api and have it up and running with debian, and the tweaked kernel put on here. I'm not familiar with uefi - which is probably the issue
I have a powered USB 3 hub and a USB SSD ...
Question 1: How do I migrate from the internal eMMC to the SSD. Just 'cp -a' or dd and change the boot drive
Question 2: How can I add multi-boot to try other OS's?
Question 3: Can the pi boot to a btrfs OS? I imagine so but would need to change something with uefi? grub?

Thanks. Handy little device, very useful for me (I needed to run an x86 program that wont run on a pi and don't want a normal power hogging PC 😀)

I don't know what that cp -a command does, but I say just change the BIOS boot order. Then set it to boot from said SSD. If you can set up GRUB bootloader it'll work just like any other PC, and enable easy multi boot. I cannot say what it is capable of booting as far as specific OSes, I'd say you have to try it to know. Personally, I got the Atomic Pi to experiment with, without risking my gaming PC with deep terminal commands.

So, I was able to get it to work - maybe. I had problems with using the uefi partition off the ssd automatically. If I press F7 and select it, it always works fine - I wonder if there's a timing issue.
Anyways, I used several links to create the partitions and copy over - with some issues.

https://wiki.gentoo.org/wiki/Btrfs/Native_System_Root_Guide#Other_volumes
https://unix.stackexchange.com/questions/558604/grub-mkconfig-gives-error-in-chroot-environment
https://askubuntu.com/questions/831216/how-can-i-reinstall-grub-to-the-efi-partition

My biggest issue was that grub tried to boot a 5.X kernel instead of 4.X - after setting a default in /etc/default/grub - when the grub is loaded from the SSD, it works great.