Unlock 1440p (2K) and 2160p (4K) resolutions in iOS YouTube app.
For a few years, YouTube had been testing 2K/4K resolutions on iOS as A/B (Alpha/Beta testing). The first group of users will see 2K/4K options while the others won't. There are certain prerequisites for those options to show:
- Whether the iOS device support VP9 video decoding, which implies on Apple's end to be at least on iOS 14, and later YouTube says iOS 15 is the minimum requirement.
- Whether YouTube decides on their end to include those options for that particular iOS device. The obviously slow devices are excluded.
YTUHD attempts to bypass those restrictions for all 64-bit devices running iOS 11 or higher.
Hardware accelerated VP9 decoder is technically added as of iOS 14 and YouTube has been utilizing it through a private entitlement com.apple.coremedia.allow-alternate-video-decoder-selection (All apps are equal is a lie).
This decoder handles up to 4K, although not all devices that can run iOS 14 get this decoder. The old iPhone SE (1st gen) is one example that hardware VP9 decoder is entirely absent from the firmware file.
Those old devices don't get AppleAVD driver which is essential for VP9 decoding to work. Such attempt to load VP9 decoder (inside AppleAVD driver) from /System/Library/VideoDecoders/AVD.videodecoder (provided that you can extract that from dyld_shared_cache of a new device) will result in AVDRegister - AppleAVDCheckPlatform() returned FALSE. There is also no reliable way to extract this decoder so that it can be successfully loaded on a device as if the binary is standalone and not embedded inside a shared cache.
If you look at the source code, there is an enforcement to not use server ABR. The author has yet to figure out what ABR stands for but its purpose is to fetch the available formats (resolutions) of a video. When the flag is set to true, it's entirely up to YouTube server to respond to YouTube app of the video formats the user can be served. YTUHD has no control over that and has to disable it and relies on the client code that allows for 2K/4K formats.
The history has shaped YTUHD to spoof the device as iOS 15 (or higher) for those running lower. The user agent gets changed from spoofing for YouTube server to respond with VP9 formats and all the goodies.
It's been reported that the sideloaded version of YouTube will not get 2K/4K even with YTUHD included. This is because of a big reason: VP9.
Normally when an app is sideloaded, the private entitlements get removed (including com.apple.coremedia.allow-alternate-video-decoder-selection) and the app won't be allowed to access the hardware VP9 decoder. There is no known solution to bypass this, unless you use TrollStore which allows for practically any entitlements, including the aforementioned, to be in your sideloaded app.