An upcoming version of the YouTube app for Android will let you set a preferred video quality for streaming. This feature was discovered in APK version 15.12.33 and given the all-caps "BETA" notes next to the new toggles, it seems that there might be a few bugs left to iron out. Currently, YouTube has a default video quality that is set server-side. Speaking of, Google decided to reduce the global streaming quality, setting the default to 480p to free up bandwidth for other uses. Of course, while watching a video you can switch to HD quality manually, but the next video will start playing...
Source: https://www.gsmarena.com/youtubes_android_app_will_get_a_default_video_quality_setting_in_upcoming_update-news-42234.php
