YouTube needs no introduction, it is as ubiquitous as television and the internet itself. Although YouTube isn't a music player necessarily, many people treat it as one because of its absolutely massive library of not only music, but of live renditions, albums, mixes, remixes, playlists, alternative performances and 24/7 live streams.
Despite its roots as a Video platform, YouTube has slowly grown toward the music industry, with for example their collaboration with Sony's VEVO publishing platform, and their recently introduced music.youtube.com platform.
YouTube, especially now that music.youtube.com exists, is one of the most popular music players of them all.
As YouTube is an online music player, you need to install the smg web plugin. After that has been installed, you can start using SMG with YouTube. Note that there is a different dropdown option for either "music.youtube.com" and "youtube" in SMG, make sure to select the one you want to use.
There are no known major issues with either YouTube or music.youtube.com at the moment, if you're experiencing issues anyway, it's likely that SMG itself is misconfigured. First, check if SMG is connected to the SMG web plugin: click on the SMG icon in your browser, and look at the connection indicator, if it's not green, then SMG is not connected. Try restarting SMG and your browser. If that didn't help, contact support at firstname.lastname@example.org, or find us on our Twitter.