Relationship between Google and Apple is little complicated. Google has high income from ads and users tracking on Internet, while Apple released Content Blockers in iOS 9 and OS X El Capitan, and later in iOS 11 and macOS High Sierra also introduced Intelligent Tracking Protection, which was improved in iOS 12 and macOS Mojave. Google apps for iOS are known to slow adoption of latest technology – Google Analytics app for iPad is still not updated for higher resolution of 12.9-inch iPads Pro and YouTube app did not get Picture-In-Picture support introduced for iPads in iOS 9.
As for YouTube app, Google in past added HDR support for the last year iPhone X, but it did not work under this year’s iPhones Xs and iPhones Xs Max. Both of these iPhones are equipped with Super Retina HDR display with support for HDR video and it was just matter of time when Google add support for these iPhones, because iPhones X were already supported for a long time. With last update of YouTube app for iOS, all owners of iPhones with Super Retina displays can enjoy HDR videos.
HDR videos offer up a broader range of colors and luminance compared to non-HDR content, just make sure that you have brightness set to the highest possible level for the best experience while watching HDR content.
iPads Pro from 2017 also support HDR content, but Google doesn’t provides HDR content for iPad Pro users. It’s unclear what leaded Google to this decision, but iPads’ big displays would be great to watch HDR movies and videos. Maybe in the future.