A stream configured for 1080p can play at 720p or another available resolution for a viewer. The source resolution and the resolution selected for playback are different: an adaptive player can choose a lower available quality to suit viewing conditions.
How automatic quality works
When the video player offers Auto, it can adjust playback quality as conditions change. For example, YouTube’s official quality guidance lists internet speed, player or screen size, the original video’s quality and browser support as factors. A lower-quality source cannot become HD simply by selecting a higher setting.
Why use Auto?
Auto is a useful starting point because a high fixed resolution can require more bandwidth and may buffer on a slower connection. Automatic selection can reduce interruptions, but it does not guarantee uninterrupted playback or the highest available resolution.
Can viewers change the quality?
This depends on the video provider, player, device and controls available for that session. On YouTube for desktop, select Settings (the cog), then Quality, and choose an available option. Other players may use different controls or offer no manual quality selector. Some formats are unavailable on certain devices or browsers. If playback buffers after choosing a higher resolution, return to Auto or try a lower available quality. A 1080p source does not guarantee that every viewer will receive or be able to select 1080p.