Most video sites operate on streaming algorithms. These algorithms only allow the online streaming of videos and prevent the downloading of videos to your disk or memory. Although, downloading videos from these kind of sites are breaking the rules. But there are lot of ways to do so.
In the case of YouTube, it allows you to save videos for offline streaming. But it doesn't allows to download the videos to local drive or memory. But there are lot of methods which let you to download videos or just extract the audio and save them to your local drive or memory. The simplest way you can do this is by removing three letters from the video URL. For example a YouTube video URL will look like this https://www.youtube.com/watch?v=Boc39rGYcrc . Now remove the letters 'ube' from the word 'youtube' on the link and hit 'enter'. This will take you to a new website called 'yout.com'. This site allows you to download the video or just the audio of the video as an mp3 file by selecting appropriate option.
Download Facebook Videos
Similar to YouTube, the Facebook mobile application also has the feature that allows users to save videos for offline viewing. Inorder to save the video, just play the video you want to download and open the three dot menu in the top right corner. From the list of options, select 'Download'. The video will be downloaded to your device so that you could watch it offline later.
If you want to download the Facebook videos from your web browser on a desktop, you can download that video file easily by the following steps. First, determine if the video is hosted by Facebook and if it is, "Right click" on the video and select "Copy link address". Open a new tab and search "downvids.net". Paste the video link, select the video format and hit the 'Download'. This site works for many other video sites and so you can even use it to download from YouTube or other streaming websites.