Apple doesn't provide a youtube app on its apple watch, so you can download third-party apps and watch youtube videos directly on your apple watch .follow this step

The apps are almost as feature filled as the official youtube apps .watch tube is completely free to download on the app store.

watch tube  currently has the following features:

1. Watching videos

2. searching for videos  

3. watch history

4. subscriptions and liking videos 

5. background play 

once the app is installed on your watch, just open it and you are ready to watch a youtube video. watch tube app doesn't require any complicated settings or even a setup process from your iPhone.The built-in search works well and you can use dication and scribble to search for a video in the app. Home shows videos that are trending on the platform.