Booting process of Linux: https://en.wikipedia.org/wiki/Booting_process_of_Linux

Booting process of Windows NT since Vista: https://en.wikipedia.org/wiki/Booting_process_of_Windows_NT_...

UEFI > Secure Booting, Boot Stages: https://en.wikipedia.org/wiki/UEFI#Boot_stages

The EFI system partition is conventionally /boot/efi on a Linux system; and there's a signed "shim loader" that GRUB launches, which JMP- launches the kernel+initrd after loading the initrd into RAM (a "RAM drive") and mounting it as the initial root filesystem /, which is pivot_root'd away from after the copy of /sbin/init (systemd) mounts the actual root fs and launches all the services according to the Systemd unit files in order according to a topological sort given their dependency edges: https://en.wikipedia.org/wiki/EFI_system_partition

Runlevels: https://en.wikipedia.org/wiki/Runlevel

runlevel 5 is runlevel 3 (multi-user with networking) + GUI. On a gnome system, GDM is the GUI process that is launched. GDM launches the user's Gnome session upon successful login. `systemctl restart gdm` restarts the GDM Gnome Display Manager "greeter" login screen, which runs basically runs ~startx after `bash --login`. Systemd maps the numbered runlevels to groups of unit files to launch:

  telinit 6 # reboot
  telinit 3 # kill -15 GDM and all logged in *GUI* sessions

You can pass a runlevel number as a kernel parameter by editing the GRUB menu item by pressing 'e' if there's not a GRUB password set; just the number '3' will cause the machine to skip starting the login greeter (which may be what's necessary to troubleshoot GPU issues). The word 'rescue' as a kernel parameter launches single-user mode, and may be what is necessary to rescue a system failing to boot. You may be able to `telinit 5` from the rescue runlevel, or it may be best to reboot.