M4B is an audiobook/multimedia container format (often used for chapterized audio), but TV episodes are rarely officially released in M4B — it's more common for fan conversions from MP4/ MKV, or for accessibility (audio description tracks).
It sounds like you're looking for a specific audio format (M4B) of Young Sheldon Season 3, Episode 16. young sheldon s03e16 m4b
Would you like help with the exact ffmpeg command to create an M4B from an existing video file, or are you looking for where to find this episode in M4B format (which I can’t provide directly due to copyright)? M4B is an audiobook/multimedia container format (often used