Online streaming sites such as YouTube or any other website, when you watch a video from there, they are downloaded to your system's cache. You can find those videos Remain in the cache which can be copied for offline viewing.
"Video Cache View" is an excellent portable tool for this job. Since it is portable so it can go anywhere you have set up.
After running on the system, it will find on the videos available in your system. With this tool, you can see how many videos are in the system, their sizes are and how they are related to the website, there are locations in the system to others.