There are several applications out there that help us to use a spare Android Tablet or iPad as a external monitor in Windows PC or Mac OSX Laptop over WI-FI or using a USB cable. For example:

• TwomonUSB
• DuetDisplay
• LunaDisplay
• Apple Sidecar
• SplashTop Wired xDisplay

Unfortunatelly there aren’t any Application for Linux, however and thankfuly to opensource community, there are articles explaining how to do similar integration based on X11, VNC, bunch of configuration and scripts, etc. That is not perfect and it’s too hard to do work it, but helps to understand the mechanism behin of commercial Applications. Below a list of articles and discussions if you want to try it yourself.

• Using Android/PC as a Second Monitor in Linux
  September 13, 2019 by Sangam
  https://sangams.com.np/using-android-pc-as-a-second-monitor-in-linux/
• Using iPad as a 2nd monitor on Linux
  Apr 15, 2018 by Bumsik Kim
  https://kbumsik.io/using-ipad-as-a-2nd-monitor-on-linux
  https://github.com/kbumsik/VirtScreen
• [Discussion] iPad as second Display on Ubuntu - 2016
  https://www.reddit.com/r/Ubuntu/comments/4lqzpq/ipad_as_second_display_on_ubuntu/
• [Discussion] Extending desktop over VNC, the easy way - 2014-12-27
  https://bbs.archlinux.org/viewtopic.php?id=191555

I hope this helps you.