Copying instead of converting
Every other option on this site decodes the audio and encodes it again. This one does not. The audio track is lifted out of the video container and written into an M4A container unchanged, sample for sample. It is the only way to extract audio with genuinely no quality loss, and because no codec runs, it is limited by how fast your disk reads rather than by your processor.
It also works on browsers that cannot decode audio at all, because nothing needs decoding. On an older iPhone or on Firefox for Android, this is the option that still works.
The one trade-off
Cuts made on a copied file land on a compressed frame boundary rather than on the exact sample you chose. For AAC that boundary is about 23 milliseconds wide. For trimming a song or a recording this is invisible; for anything that has to line up precisely with something else, convert to MP3 or WAV instead and accept the re-encode.