Whenever you start playing any video, it usually starts playing in "auto" mode.
The clarity of a video in "auto" mode depends on the internet speed being received.
You would however be able to change the resolution manually upto 1080p.
If using a mobile app, while playing any video click on the settings icon available at bottom left, followed by "playback quality" to choose the desired resolution.
If using the website or TV app, while playing any video, click on "auto" available at bottom right followed by the desired resolution.