It’s a very easy task and there are so many ways you can come about this.
1. At first, videos from YouTube can be downloaded directly to your phone simply by clicking on the three dots at the bottom side of the particular video you want to download, once you click on the three dots, a drop down menu will appear, then select and click on ‘download videos’ and your download will start immediately.
2. The above described process can only download video on YouTube and not directly to the phone memory, to download directly to the phone memory, copy the URL of the video from the top which usually looks like http://www.youtube.com/……, after you have copies it, open a new tablet and paste this link there and this time, add ‘ss’ between www. and youtube.com as in, you will transform it from http://www.youtube.com/…… to http://www.ssyoutube.com/…… And your download will start immediately to the phone memory.