Streaming is not always one file
When a video plays smoothly at several quality levels, YouTube is not necessarily sending one giant file. Modern delivery can split the picture and sound into separate streams and adapt them to the connection. One stream may carry a 1080p image while another carries the matching audio track.
The role of yt-dlp and FFmpeg
That design is excellent for streaming but confusing for offline files. A video-only stream has no sound, and an audio-only stream has no picture. Tools such as yt-dlp inspect the formats, select compatible video and audio, and use FFmpeg to mux them into a container such as MP4 or MKV.
Muxing versus conversion
Muxing is not the same as re-encoding. In a clean merge, streams are placed together without unnecessarily recompressing every frame. If codecs do not fit the chosen container, a conversion step may still be needed. YGet hides this technical layer behind a result list so you can choose compatibility, smaller size, or quality.
When compatibility matters
If a result has picture but no sound, or does not play on an older device, the cause is often stream or container compatibility rather than a broken YouTube link. Try a more compatible result in YGet, or choose MP4 when broad device support matters most.