Find a site with a download link for a video and press the link. That's the "process". It doesn't matter whether it's a text file or a video, you're downloading a file.
As far as sources for particular size videos, I can't help you. I don't download (or watch many) videos.
As a side note - downloading (as opposed to watching by streaming) from Youtube is a violation of Youtube's Terms of Service that you agreed to the first time you went there, even though there are apps on the Play Store to download Youtube videos. Left hand, right hand, you know.