It's the Youtube app. It's bs in terms of coding.
I had this problem with my iPhone and now on my Samsung s10e. Where the app just plays in 480p or 720p (at best), if I set it to automatic resolution.
Despite having full HD screen (or on iPhone at least above 480p).
The app just doesn't care.
You need to set it manually sadly.
It doesn't care if you have: full HD screen and you are on 150Mbps wifi.
Hère is the solution : You have to turn on the 4k definition of your phone (it's not done automatically).
Go to : Settings > Display > Select "WQHD+".
Now, your phone is displaying in 4K.
You can now watch 4K video on youtube.
All your experience will be now in a better resolution, apps, photo, video, etc. 3040 x 1440. (Yeah, many people don't fully use their phone).
Without the highest resolution, photos and videos don't have same/highest quality. As all your posts on social media like Instagram. This is very important to know for community managers, i think.
We all know that. Simply because it starts with a "3" (3040). Every 4k monitor aren't. Youtube call it 4k, so, i follow the river and i say 4k.
YouTube 4k is 2160p
The samsung devices can only display 1440p, they are not 4k. That's just fact.