X.app features 2 extensions for changing video mode:
App Store download link:
Picture-in-Picture Video Extension activates PiP in all apps on iPhone and iPad:
Besides Picture-in-Picture Extension, X.app also contains a Fullscreen Video Extension, which has 2 purposes:
App Store download link: https://apps.apple.com/app/id1533525753
Sites and apps like YouTube, Twitch, Vimeo, Facebook and others use custom fullscreen interfaces on iPad.
Websites provide HTML based interfaces which are less responsive compared to native iPadOS fullscreen UI and lack useful features.
YouTube HTML fullscreen interface:
X.app (2021.4.21) for iPhone, iPad and Mac is available on the App Store: