For Android, you have been able to check out Youtube in 4K resolution. Now it looks like the same feature has come to iOS with the latest version of iOS.
So it looks like you will be able to watch videos from YouTube in 4K resolution if the video supports this resolution. After all, it feels good and alternative when it comes to YouTube.
First: MKBHD video, maxed out at 1080p, AVC1- Cube (@CubeCDM) June 24, 2020
Second: 4K60 HDR video, maxed out at 2160p60 HDR, VP9
Third: 8K60 HDR video, maxed out at 2160p60 HDR, VP9
Seems like iOS Youtube finally supports high res playback for VP9 codec pic.twitter.com/bfQr76WMGn