Frank Slootweg wrote:

Another example: One of my files which has a .mp4 extension:

- has a MPEG4 container with:
video: H.264 encoding
audio: LC-AAC encoding

So this file has a .mp4 extension, but the video encoding is *not*
MPEG4 and the audio encoding is *not* MP3.

H.264 = MPEG-4 part 10 AVC
https://en.wikipedia.org/wiki/H.264/MPEG-4_AVC

There is also:

H.263 = MPEG-4 part 2
https://en.wikipedia.org/wiki/MPEG-4_Part_2

There are multiple naming conventions for the same coding and
compression format: MPEG, ISO/IEC, and ITU-T. See:

https://en.wikipedia.org/wiki/MPEG-4

AAC is the successor to MP3 and probably why it shows up in an MPEG
container file. Not sure why you thought an .mp4 container must or
hints at MP3 for audio encoding.

LC-AAC, AAC-LC = MPEG-2 ACC aka MPEG-2 part 7 aka ISO/IEC 13818-7.