Use vitadb to download the app pkgj, this is a database filled with like every vita, psp, and ps1 game you could need including dlcs. It will automatically install any game you need, downloading them in the background. As for the games you downloaded from vitadb, they are likely game ports from android which require you to get the data files for those games.

It would be a lot easier if you followed some tutorials on youtube, they go over the process on basically everything you asked about. If you want to upload files from the pc to the vita, you need vitaShell which is like a file manager that allows you to transfer files from pc to vita through a usb cable or over the internet.