You have to have brand registry first to load videos to Amazon Seller Central. If you need more information about brand registry click this video first.
To upload videos go here: https://sellercentral.amazon.com/creatorhub/video/library/
Video Tutorial to load videos to Seller Central
Once you load the video it will show up in the photo section on the top left of an Amazon listing.
Videos are not clicked on much on Amazon. However, they increase conversion rates.