M4A = AAC in an MP4 container
M4A is the native audio format used by Apple Music and the iTunes Store. At any given bitrate, AAC sounds noticeably better than MP3, especially at lower rates. For iPhone, iPad, Mac, or HomePod, M4A is the smarter default.
Direct mux where possible
YouTube's native audio stream is already AAC for most videos. When that's the case, we simply re-mux the AAC bitstream into an M4A container without re-encoding — zero generation loss and near-instant processing.
Frequently Asked Questions
Yes. Android has supported M4A/AAC natively since version 3.
The video thumbnail is embedded as cover art when the source provides one.