Now a days, it is super simple to download videos from youtube..
Whatever video you want to download, just add ss after www. And before youtube.com and then dial it in browser it will redirect to savefrom.net it will show you video quality options to download video.
See example here
Or you can directly goto savefrom.net and paste youtube video link to download.
You can download mobile apps to download YouTube videos directly while watching, in audio or video format..like