How can I share my secondary monitor to a remote system as if it were directly connected to it?

[kovacslt (László Kovács)]

For other visitors: my wrong reply had a link to my previous experiment:

Mirror (extend) desktop to KODI over LAN (xrandr / ffmpeg exercise)

I have a Raspberry 3B+ in my living room, which serves a bunch of functions. One of its purpose is to be my media player (attached to TV via HDMI), so it runs KODI on Raspbian Buster. Now another purpose for it: display something “projected” from a computer directly. I intended it for looking at family photos on the TV. So my solution which involves xrandr and ffmpeg as Lego bricks to play with… First I create a virtual monitor on the computer, then set it to a desired resolution (FullHD): …

The tools I used here are xrandr and ffmpeg.
I have 2 monitors on my desktop, and a regar laptop so I may fiddle with them to bring something useful for you. However, I’m far away from my computers until september, so I just visit here using my phone.