Scanning with Android There are a few apps that seems to act like document scanners?

How do they work?  Is it more than a lot of posprocessing of one shot?

Are they actually "scanning" or utilizing more several shots?

What kind of phone specs do I need to have quick and reliable document scanning?
In my opinion, the word scanning in this context is a misnomer.
Basically the camera and the flash are the key hardware components involved in this operation.  The camera doesn't go into burst mode and instead takes only one good shot. The rest are the magic of software algorithms that straighten, finds boundaries and brightens and de speck the image.

The app Camscanner supports creating documents either from image or capturing with camera.

Higher camera resolution, the ability to autofocus and presence of flash (in absence of ambient light) are the features that are to be looked for. For shooting an A4 size with text printed with font size of 12,  a 5 mega pixel resolution is needed and with 3.2 mega pixels, I am able to capture A5 sized papers with very decent quality - observed from my experience.