Well, it all depends on where do you want to download them from. If you want to use torrents then search for that specific format if you want to download a video from Youtube then you would have to use a tool like these ones:https://www.makeuseof.com/tag/18-free-ways-to-download-any-video-off-the-internet/Those two will show you how to download and convert to the desired format and size.If what you want is to rip a DVD to watch later on your phone then check this link:https://www.makeuseof.com/tag/5-simple-steps-rip-entire-dvd-hard-drive/Hope it helps.