By default, YouTube doesn't provide a way for you to download the videos and instead you need to use Chrome Extensions from the App Store. You need to click on the
Add to Chrome button then restart the browser for the settings to take effect. Upon loading a new video on the Youtube you will have the option to download it. Contrary, I suggest on using a web based service which helps you download Youtube videos plus the output file will have the enhancements you select when you decide to download the video.