You cannot use a YouTube app directly on the display as it is a portable wireless screen for CarPlay and Android Auto that mainly mirrors the functions of these systems. YouTube videos are usually not directly playable in CarPlay and Android Auto for security reasons.
The device displays content from your smartphone via CarPlay or Android Auto, although video apps such as YouTube are usually not supported to avoid distraction while driving. If you want to watch YouTube, you would have to do this directly on your smartphone, not via the display.