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.
What is MP3?
MP3 is the format that made digital music portable, and thirty years on it is still the safest bet that a file will simply play - on any device, in any car, in any decade-old piece of hardware. It is lossy, and its patents expired in 2017.