[snowwrestler]

You won't find it with those search strings because the American viewer (presumably who is posting those videos to YouTube) takes it totally for granted.

Try searching for just "first down" with football, or viewing some game highlights on NFL.com.

[clarky07]

I was about to say the same thing. You wouldn't even need to search first down, just watch any football footage from the last decade. The first down line is going to be in pretty much every shot. Note that it isn't always yellow. Sometimes it's orange, and there are also other lines, namely the line of scrimmage, that are marked as well.