x11vnc gives you the same session the user sees

check whether x11vnc package is installed:
sudo dpkg -l | grep x11vnc

install x11vnc if needed:
sudo apt update && sudo apt install x11vnc -y && sudo apt clean

set up the x11vnc password:
sudo x11vnc -storepasswd /etc/vncpasswd
sudo chmod a+r /etc/vncpasswd

LightDM
-------
create configuration
/etc/lightdm/lightdm.conf.d/20-ubuntu.conf
[SeatDefaults]
display-setup-script=sh /etc/x11vnc.sh
session-setup-script=sh /etc/x11vnc.sh

both scripts with same content:
/etc/x11vnc.sh
#!/bin/sh
killall x11vnc &>/dev/null 2>&1
sleep 2;
xhost +localhost
/usr/bin/x11vnc -rfbauth /etc/vncpasswd -rfbport 6000 -nowf -forever -shared -norc -notruecolor -scale 4/5 -scale_cursor 1 -desktop name -bg

.... replace "name" with any name that describes the host what you would like to see