[TLDR] GRUB, dracut, systemd and Bootup Process

BIOS POST: issues interrupt INT13H to find boot sectors on bootable device
Boot Loaders (LILO / GRUB / GRUB2): loads Linux Kernel into memory and running
- stage1: runs bootstrap code that loaded by BIOS POST in Master Boot Record (MBR)
  - stage1 does not understand file system, the only purpose of stage1 is to locate and load stage1.5
- stage2: locates the stage 2 files in the /boot filesystem and load the needed drivers.
  - understands common filesystem drivers, e.g. EXT, FAT, NTFS, etc.
  - stored in core.img
- stage3: locates and loads a Linux kernel into RAM and turns control over to the kernel
  - located in /boot/grub2
  - runtime kernel modules located in /boot/grub2/i386-pc
Kernel: located in /boot along with initramfs image and device maps of the hard drives
- de-compressed in memory
- executes initrd, an initial RAM disk image generated by dracut
  - note: The initramfs has (basically) one purpose in life – getting the rootfs mounted so that we can transition to the real rootfs.
- systemd: PID=1, init process, daemon
  - responsible for probing all remaining hardware, mounting all necessary file systems and spawning all configured services.

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