Linux boot starts with BIOS/UEFI initializing hardware, then GRUB loads the kernel and initramfs, the kernel initializes the system, and finally systemd starts all user-space services.
Power ON ↓ BIOS/UEFI ↓ GRUB (Bootloader) ↓ Kernel (vmlinuz + initramfs) ↓ Init (systemd) ↓ Login Screen / Shell