It is youtube-dl
If you are Windows user, there is an executable file is available in the following site. Download it from here.
https://yt-dl.org/latest/youtube-dl.exe
If you are using, Ubuntu or Debian based Linux system use the following command to install the software from the Terminal.
apt-get install youtube-dl
After the installation process done, please copy a youtube share link to do your first experiment. For an experiment, here is a link - https://youtu.be/L6-uJLteKek
We're now just going to download "Mr. Big - To Be With You (MV)"
youtube-dl https://youtu.be/L6-uJLteKek
After giving the command in the Terminal, just sit back and watch the verbose happening in the screen. Once the completion status reach 100%, check it in the current folder in the file manager.
Ok. You have downloaded the first Video file from youtube. Good. But when it will come to download multiple files and convert them into mp3 files, what will you do? The command is very simple. Now you are just going to copy a youtube play list link. Here is the example case. The Beatles collection - https://www.youtube.com/playlist?list=PL9-cZf_sidpkzR4W_LxvZjh4F7YFo4WoG
And here is the command to download all the files as mp3.
youtube-dl -i --extract-audio --audio-format mp3 -o "%(title)s.%(ext)s" https://www.youtube.com/playlist?list=PL9-cZf_sidpkzR4W_LxvZjh4F7YFo4WoG
Now just sit back and relax. It will convert all the files to mp3. So easy, so simple.