Introduction to Media Containers
A media file (.mp3, .mp4, .mkv, .webm, .ogg, .opus, .avi) is a container for media.
An .mp3 file has the audio stream and optionally descriptive metadata.
An .mp4 file can hold any number of
- audio stream
- video stream
- subtitles
- metadata
A Matroska .mkv file can hold any number of
- audio stream
- video stream
- subtitles
- chapters (referencing sections within the file or in other files)
- metadata
You can inspect a file with ffmpeg -hide_banner -i <file>. (Or ffprobe -hide_banner <file>.)