Why does the YouTube video opens in the YouTube app on mobile instead of the lightbox like on desktop?
A lot of mobile phones have limitations about showing iframe content. Some of them won't show them at all, so the video wouldn't show up inside the lightbox and you would see just an empty lightbox. Other phones are opening the Youtube app automatically instead of showing the video in an iframe so if Smart Slider 3 would show the YouTube videos inside a lightbox, it would be empty again.
To avoid such problems Smart Slider 3 automatically opens lightboxed Youtube videos inside the Youtube app, not inside the lightbox on phones.