My understanding is that the UEFI/BIOS will boot into a certain storage device, whether it be HDD, SSD, USB, DVD, etc. Then this will pass control to the boot-loader, which is GRUB in Linux operating systems. (additional question: do other Linux OS's require different bootstrap loaders?)
Does GRUB load an OS from a specific partition on the drive? If so, how can I direct it to boot from a specific partition? Will installing certain ISOs in certain partitions using GParted work? I am currently running Ubuntu on a USB SD Card Reader with a MicroSD card temporarily.