If i scroll my 3 rd project the video overlaps the nav

if I scroll my project everything goes good but the video overlaps the nav how I fix it?
here is my code. please scroll it until the end?

Give the #nav-bar a z-index (e.g. z-index: 10)

Thank you it works when I set (z-index: 10` for nav ) but I am confused. because I set z-index only for nav. how he knows that this z-index is higher than the video content. I didn’t set z-index for video.