NOTE: You have to use Wayland, not Xorg.
gsettings set org.gnome.desktop.remote-desktop.rdp screen-share-mode extendNext, open the Settings app, navigate to the Sharing panel, and turn on Sharing, and turn on (once it appears) the ‘Remote Desktop’ option. You will want to set up a username and password too, to allow you to connect from a different device.
Use an RDP client on your tablet/phone
Finally, head back to the Settings > Display panel to reposition your virtual monitor, configure the screen resolution, etc. Remember: any multi-monitor behaviours you’ve tweaked elsewhere (e.g., ‘show dock on all monitors’) are respected by virtual monitors too.