How video formats work
When YTSave shows a list of formats, it is reflecting what the source already stores. The labels are not invented quality buttons.
Container vs codec
A container (.mp4, .webm, .mkv) is the box. The codec (H.264, VP9, AV1, AAC, Opus) is how the picture and sound are compressed inside that box.
Phones and TVs are picky. H.264 video plus AAC audio inside MP4 is the most widely playable combination. VP9 or AV1 in WebM can look good at a smaller size but may fail in older players.
Why the list changes
Some sites offer one render. Others offer a ladder: 360p, 720p, 1080p, and sometimes higher. Audio may be a separate track that the downloader merges when both exist.
If MP4 is missing, the source may only have WebM. YTSave does not silently re-encode every file into MP4.
Audio-only options
If a video has a soundtrack, you may see M4A/AAC (often the original) and a converted MP3. Silent clips have nothing to extract.