Back in the Master Boot Record (MBR) days, the boot loader was just in the boot sector of a partition and you could mark it as active with fdisk. So you could move a disk and it would "just work." But now with UEFI and GUID Partition Tables (GPT), the BIOS has to be told: add a boot entry labeled Linux that points to (hdx,x)/EFI/Grub.uefi or whatever. You'll see that at the bottom of my guide.
If your motherboard supports UEFI shell (there's usually a "Start UEFI shell" option. Sometimes its built in to the BIOS, sometimes you have to put the shell on a USB stick), you can use that to navigate to the UEFI executable. The command syntax is similar to MSDOS. You type fd0: <enter> dir / cd.... You can find your bootloader and start it, then use efibootmgr after you're booted to add the entry permanently.
Some BIOS setup screens let you configure the UEFI menu (Dell, Framework, etc.) It's a nice feature but not very common.
Yes.
\efi\BOOT\BOOTX64will boot without needing to be set in the BIOS. (x64 can be replaced for different architecture) That's how bootable USB sticks work.