I'm using "HD Downloader" on my Android phone. It's a bit advert heavy but usually works quite well and can download to a location of your choice in MP3 or various video formats. Problem is I've just done a search of Google Play looking for an app of this name with the correct logo (red background with white circle containing solid single down arrow pointing to a tray)(grphics very similar to the logo on this page https://play.google.com/store/apps/details?id=com.turbo.videodownloader but this is not the same app) and cannot find it at all! It might have been on my phone at the start which suggests it comes with Android. My phone is a UK provider "own brand" (EE "Harrier", made by Benq) with a pretty much standard install of Android 5.x.

On my PC I use a 2 stage process, firstly get the video using "Youtube Downloader HD" which is all over the place including http://www.softpedia.com/get/Internet/Download-Managers/Youtube-Downloader-HD.shtml and then use WinFF to convert to a suitable format. WinFF also on SoftPedia http://www.softpedia.com/get/Multimedia/Video/Encoders-Converter-DIVX-Related/WinFF.shtml