This topic has been brought up multiple times in this forum over the years but doesn’t seem to have a current solution.
Any video player, whether or not it is youtube/vimeo/video tag has fullscreen disabled when working with the embedded application. This is likely due to the permissioning on the iframe itself set by Shopify. There have been multiple topics that have asked the same question but no answers have been provided. Here are a few references.
https://github.com/Shopify/polaris/issues/231
Has anyone found a way to enable fullscreen on video players or found a good workaround?