The youtube is one of the apps installed by default in KaiOS. Check all functionalities in this analyze.
The pages of youtube is basically a responsive version of web page of youtube. The navigation is made with pointer using d-pad. The pointer navigation is not the best way to navigate in KaiOS devices, but works well in this app.
Basically clicking in video it open the player and after the video load automatically open in horizontal mode:
You can interact with the video using d-pad to forward, rewind and pause.
Other nice functionality, is the search, wicht you can use de voice to make the searches: