The website you are referring to are probably using the MediaSource API (for instance YouTube does, that's why they use a blob:// URI).

This API allows to "compose" the media to be played using different sources. This is how most adaptive streaming is done. When the bandwidth can't cope anymore, the script will request a lower quality and push it to the end of the MediaSource, just like if it was the same file as the highest quality that got fetched until then.