1) YouTube uses their own proprietary VP9 codec for 4K video, so, you won't see 4K, nor HDR, as virtually everyone else is using HEVC/H.265 for 4K & HDR. Unless Google updates the YT app, which they can, but probably won't, don't expect this from YT. This is Google's problem this time, not Apple's. Be clear, this is Google's issue this time.
2) In order to utilize HDR10 & DolbyVision encoded content,
--for iTunes, you simply need to update to iOS 11.
--For Netflix, iOS 11, update the app, pay extra for Premium subscription.
--VUDU has not updated their app for 4K/HDR yet.
--YouTube, again, Google went proprietary with their 4K HDR content using VP9 codec. The problem is not with Apple, but, Google. Don't blame apple, blame google.
--Did I miss an app someone's asking about?
3) From what I've seen, your iOS device needs to have an A10X or A11 chip for HDR10 & DolbyVision. Only iPhone X is touted as having a true HDR display, as it is OLED and has much more contrast (and blacker black), however, the Video Playback specs for iPhone 8/8+, and 2017 iPad Pro models specify they support HDR10 & DV content.
Netflix updated the iOS app for HDR on Sept. 21st, and bug fixes yesterday:
*If you don't see this last line on an app update, it probably does not support HDR playback ("HDR video (on supported devices...).)