Welcome to the Bubble community! When you send the data to the popup you can reference the videos data source from the popup. Then when closing out the popup with an X just reset the popup data and the video stops. If you need an example just let me know.
If you want more help with Bubble I do one on one tutoring sessions as well. Take a look at my site if you would like to schedule a session www.NoCodeMinute.com.
Most of my students accelerate their learning very quickly with just a few sessions. Some of them even get their first version of their project released within a month or so.