Having a secondary screen has its benefits, but buying a second monitor may not always be a feasible solution. If you have a smart device lying around, say like an older tablet, you can now turn it into a secondary monitor for your PC using Deskreen. A free open-source app, Deskreen lets you use any device with a web browser as a second screen over Wi-Fi. Best of all, it offers support for Windows, macOS, and Linux.
Right now it still does need a cheap Dummy Display Plug (to make it think that an external display is connected) but they are asking for coding assistance to try and eliminate the need for the plug.
See Deskreen turns any device into a secondary screen for your PC
Deskreen is an open-source app that lets you use a tablet or phone as a secondary display for your PC over Wi-Fi. Check it out!