BIOS vs UEFI & the Boot Chain

When you power on a machine, firmware runs first and hands control to a bootloader.

Feature	BIOS (legacy)	UEFI (modern)
Partition scheme	MBR (max 2 TB, 4 primary)	GPT (huge disks, 128 partitions)
Boot data	Boot sector (first 512 bytes)	EFI System Partition (`/boot/efi`, FAT32)
Secure Boot	No	Yes (signed bootloaders)
Interface	16-bit text	Graphical, mouse, network

Boot chain (UEFI + systemd):

Firmware (UEFI) → GRUB2 → vmlinuz (kernel) + initramfs → systemd (PID 1) → targets

initramfs / initrd is a temporary root filesystem in RAM that contains the drivers needed to mount the real root filesystem (e.g. LVM, RAID, encrypted disks).
Check firmware mode: if /sys/firmware/efi exists, you booted in UEFI mode.