There are many online and offline tools that can be used to download Youtube videos. Then how do I download Youtube video from Android phones?, You can use TubeMate Android applications.
TubeMate not only provides an easy way to watch Youtube videos of Android devices, but also simultaneously function as an Android application for downloading videos from Youtube.

TubeMate:
Youtube video download streaming video as well. You can also watch another video, while TubeMate are downloading your favorite videos.
Pause and Resume feature is available as download video.
Download video in MP3 format (audio only).
Download Youtube videos in High Definition format (HD).
File sharing in social media.

Download and install Tubemate from this link