Boot Process
A CPU can only run the instructions in its instruction set. The only question is who controls which instructions run.
What happens at boot
At boot, the CPU runs instructions from a fixed address. That address is the BIOS.
Next, the BIOS loads the bootloader into memory. The CPU then runs the bootloader.
Next, the bootloader loads the kernel into memory. The CPU starts to run it. The kernel then takes control of the CPU. It runs its instructions, sets up the devices, and makes the system ready for userspace apps.
When we say memory, we mean only RAM.
ROM and disk devices are always called storage.
First steps after boot
kernel loads initramfs and executes the /init script inside it.
The kernel detects the hardware and loads all the drivers at boot. It hands over to initramfs only after these steps.
When initramfs loads, it also checks a standard location for drivers and loads them too.