I know how to do that with Sway and swayvnc on wayland, as well as with xorg.
https://superuser.com/questions/1434779/using-a-tablet-as-a-...
https://www.reddit.com/r/swaywm/comments/k1zl41/thank_you_de...
This is particularly helpful to people using asahi linux and wanting to connect an external monitor. That can be done with something as simple as a raspberry pi.
EDIT: apparently this project help achieving the same thing on different OS but using webRTC and a browser:
You'd install the client on your computer:
Windows: https://anydesk.com/en/downloads/windows
Mac: https://anydesk.com/en/downloads/mac-os
and then on your phone:
Android: https://play.google.com/store/apps/details?id=com.anydesk.an...
But I share your want of an quick and easy way to use my phone as a keyboard and mouse for a "remote" computer in the same room as me
This one was usable but not very polished IMO https://play.google.com/store/apps/details?id=com.panda_nood...
Like, no matter what, the UX will probably be meh just trying to mirror screens. Instead the presentation app could give you a QR code with a TLS cert in it, and you could scan it and get a secure connection to it's mobile optimized web remote UI.
We are really nowhere near using the full potential of all the devices and connectivity we have.
Specifically I would love a way to use my Android device as a remote controller for PDF presentation. For now I can do this with KDE Connect but the remote controller is blind, I have no way to see the current slide on the Android device, much less to use it to annotate the slides in live.
Does that exist?