Youtube lets you A/B test thumbnails as a creator and see response rates, for instance.

https://support.google.com/youtube/thread/393332200/you-can-...

A small number of creators have had testing tools provided by YouTube for years.

He also changes the thumbnails and titles of videos once published, sometimes up to dozen times in the first day.

He also has dozens of channels for different languages, so can test thumbnails and other tweaks with those.