Grub

Single-user mode vs Multi-user mode

multi-user mode just starts more systemd services that lets other users login like:

sshd - allow ssh remotely
getty - daemon that puts login prompt
NetworkManager - networking

single-user mode systemd doesn’t start these services

just a root shell

these are stored as .targets

Commands

# symlinks to all the units that need to be run to reach the target
ls /usr/lib/systemd/system/multi-user.target.wants/
# single user doesn't have a directry of services, its contained in the service file
cat /usr/lib/systemd/system/rescue.target 

# the actual target files
cat /usr/lib/systemd/system/multi-user.target 
cat /usr/lib/systemd/system/rescue.target

Booting into each mode with Grub

# highlight kernel, press 'e' to edit, append to the linux line, Control-X to boot
linux /boot/vmlinuz-6.1.0 root=/dev/sda1 ro quiet splash                    # normal
linux /boot/vmlinuz-6.1.0 root=/dev/sda1 ro systemd.unit=rescue.target      # rescue (single-user)
linux /boot/vmlinuz-6.1.0 root=/dev/sda1 ro systemd.unit=emergency.target   # emergency
linux /boot/vmlinuz-6.1.0 root=/dev/sda1 ro init=/bin/bash                  # bypass systemd entirely