Not all video delivery works the same way under the hood. Two common approaches are progressive download and DASH (Dynamic Adaptive Streaming over HTTP), and the difference explains a lot about why some downloader tools are simpler than others.
A progressive file is exactly what it sounds like: one file containing both video and audio tracks muxed together, playable and downloadable as a single unit ā much like an older-style mp4 you'd save directly to a hard drive. It's simple to work with, but it doesn't adapt well to changing network conditions and typically caps out at more modest quality levels.
DASH splits things up. The video track and the audio track are stored and served as separate streams, often further broken into small time-sliced chunks at multiple quality levels. This is what lets a modern player switch resolution mid-playback without a rebuffer, and it's how most high-quality streaming (1080p and above) is delivered today.
The catch: if you want to save a DASH-delivered video as one normal mp4 file, the video and audio streams need to be downloaded separately and then muxed (combined) back together, usually with a tool like ffmpeg. That's an extra processing step progressive downloads don't need ā which is why many lightweight downloader tools intentionally request the progressive version of a video when a site offers one, trading a bit of maximum quality for a much simpler, faster download. ## More Bilibili Download Guides
For more practical Bilibili downloading tips, see these guides: