Well. Bad news: from what I know and could see online it is not possible for these kind of tablets to use their screen as a 2nd monitor. It's not really meant to be one. The only way to do this is through apps or even just through windows' own mirroring application
Do you already know how to do this? Or should I show you?
Sorry for the late reply though, I had it very busy and kind of forgot about it too.
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Windows 10's wireless 'Connect' functionality works great. I can send a fluid 1080p image from another computer to this, wirelessly. There's a quarter-second of delay, though. I also wonder if newer computers can send more than 1080p? (I was casting with a computer from 2013.)
I'll also try and use my Chuwi tablet screen as second monitor thru HDMI cable + mini HDMI adapter for the tablet. I'll try and connect it my Chuwi to my brother's desktop PC.. Then I'll try and use the desktop PC's Wndows 10 project-to-another-screen app/buttton in the Windows 10 action center in the lowest rightmost part of the windows 10 desktop to try and extend the screen of the desktop to display on my Chuwi tablet. Will update y'all.