So adding it to the jquery.magnific-popup.min.js as described here:

https://kriesi.at/support/topic/enfold-new-light-box-magnific-popup-you-tube-video-urls/

seemed to work. I just used Filezilla (took me a while to figure out how to use it but it worked). Thanks for trying to help! If anyone else wants to fix this issue I recommend downloading Filezilla and changing the file as explained in the above link. Here is the comment that helped:

Hey!

Unfortunately the magnific popup plugin is quite limited when it comes to youtube urls. The embed url is harcoded in the js source code and you can not change it with the video element or wordpress settings. If you want to add additional parameters like the rel=0 parameter open up wp-content/themes/enfold/js/aviapopup/jquery.magnific-popup.min.js and search for:

//www.youtube.com/embed/%id%?autoplay=1
Add your custom parameters to this url – i.e. like:

//www.youtube.com/embed/%id%?autoplay=1&rel=0&autohide=1&showinfo=0&fs=1
and save the jquery.magnific-popup.min.js file. Afterwards the lightbox should load the right player based on the parameters you added to the url.

Best regards,
Peter

Here you go.

Thanks so much for your help so far!