The YouTube app update last night to v5.7.36 that includes one important new feature -video quality playback. In the recent, YouTube on cellular would only allow you to toggle between standard & high quality streams, but with the new update, you can yourself select anything from 144p up to 720p. An Auto mode exists also, just like it can do on the web, so assuming your connection is good, I would consider that it will default to 720p.
There are also choices for 240p and 360p.
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