What is M4A?
M4A is AAC audio in an MP4 container - what iTunes sold, what Apple Music and most podcasts ship, and what a voice memo becomes. At the same bitrate it sounds better than MP3, and it is read by rather less hardware.
What is OPUS?
Opus is the best-sounding lossy codec at low bitrates by a comfortable margin, which is why it carries the audio in nearly every voice and video call you make. It is royalty-free, it handles speech and music equally well, and it is usually wrapped in an Ogg container.