Over the weekend, MacRumors forum users noted that the most recent YouTube App update added HDR support for theiPhone 11 andiPhone 11 Pro. YouTube has supported HDR on its iOS app since the iPhone X but has required updates to support new hardware.
To check to see if you are watching YouTube videos at their highest quality, you can simply press the three dots on the top right of the video, and you should see "HDR" options listed on supported videos. HDR options will also only be available on devices that support HDR.
Related Roundups:iPhone 11, iPhone 11 Pro
Buyer's Guide:iPhone 11 (Buy Now), iPhone 11 Pro (Buy Now)