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Samsung tablets can act as second screens for windows PCs AFAIK.

For Linux I've had some success with Microsoft's RD Client android app -> remote to Linux PC.

I'm on gnome though, which has some experimental setting which allows you to set remote desktop clients to act as a new display instead of cloning your existing display.

I've got a top of the line Samsung tablet and a recent windows PC. The second screen experience is absolutely terrible in many ways, the worst being inconsistent lag, sometimes measured in seconds. I do not recommend it at all.

I've found it easiest to use something like deskreen. That way you don't need to rely on your tablet supporting hdmi input, you just need a browser.

You'll need a hdmi dummy plug on your PC though.

Probably, as long as your tablet supports an external webcam.

You'll need an app to view the camera. Search Google for 'android uvc viewer'.