watched video copy in the system

Many years ago, after watching a video on youtube, I went to the / tmp folder and found a copy of the watched video.
But this is not the case today.
I wonder if I can still find temporary copies of watched videos.
Does anyone know if this is possible?

Microsoft Internet Explorer in Windows until Vista did that - It was removed because malware was hiding in the IE temp folders.
Neither Firefox or Chrome keep the streaming videos any more. Youtube video downloader is the way to save the videos.