Google Opinion Rewards will periodically send you short surveys (usually 3 or less questions) and reward you with Google Play credit in the form of $. The first survey you complete (shortly after installing and setting it up) will give you $1, and subsequent surveys will award $.10-$.50 on average. The credit is immediately added to your Google balance, which can be used for anything from Play Store purchases, to in-app-purchases.
Not really, civilians use it. I personally find it easier than the am/pm silliness, especially if you are going through different time zones, but I grew up with it so I am a bit biased.
you don't need a computer, I used my phone to do all of that. you just need a file manager. this is the one I use, you can navigate, copy, paste, delete, compress, etc. https://play.google.com/store/apps/details?id=com.alphainventor.filemanager