In a nutshell one way to do this is to use a set state. Here is a video on set states if you don’t know how to use them yet. Set States FREE Video Tutorial 🎉
So you can set a state on the page itself to say “Current Video” and then when the user clicks on the list of videos on the right you can pass that videos YouTube code to the “Current Video” set state.
Then on the main video on the left just reference the “Current Video” state on the page. It will change depending on whatever the set state video is.
Nope, just one state. That one state changes depending on which video you click. Your main video only needs to look at the one state to know which one to show.