Youtube Downloader Chrome Extension
This allows Google Chrome users to download and convert YouTube videos to mp3 (320) kbps files and also videos with a variety of high quality options 2160p (4k), 1440p (2k), 1080p (HD), 720p (HD), 480p, 360p, 144p etc. This video quality is supported for all available video formats (MP4, WEBM, 3GP).
Follow steps 1 to 3 below for installation

Step 1: Install Tampermonkey

Download Tampermonkey

Step 2: Install Youtube Downloader

Then the install dialog will appear, please install it

Download Youtube Downloader

Step 3: Make sure that you use the new YouTube Layout
Visit to check and update your YouTube Layout.

Done, you will find the download button on all youtube pages like the picture below.