Why can a 1080p video on YouTube look like 240 or 360p?

Why can a 1080p video on Youtube look like 240 or 360p?

Do they put 240 or 360p video at 1080p, therefore it says 1080p but looks 240 or 360p?

Why can a 1080p video on YouTube look like 240 or 360p?
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