How do you start up Linux operating system?
On a BIOS-based machine, you configure your bootloader (Usually
GRUB.) to boot it. It will load a kernel, initramfs image, and pass
along the kernel command line and execute the kernel, which will
handle the rest of the Linux boot process.
On UEFI-based machines, it can either work the same way as BIOS
machines (Bootloader configuration.) or the kernel can be loaded
directly via the EFI Boot Stub feature, which means the system
firmware can directly load and execute the kernel. In this case
there needs to be a way for the initramfs and kernel command line
to be passed. The "official" way involves building both right into
the kernel, which has the downside of any configuration or early
userspace changes to recompile the kernel to work. The other way is
to use a boot manager like rEFInd which can pass a bootstubbed
kernel what it needs for boot almost as if it was a bootloader.