OPUS sampling rate, max playback rate and max bandwidth

How does opus sampling rate, max_playback_rate, max_bandwith is co-related and how does it affect the transcoding and voice quality?
If we set up two asterisks at narrow bandwidth (or max_playback_rate) then why voice quality is bad or is it expected that if we change the max_playback_rate to 8000 or well below 48K then voice quality will be bad?
48k sampling rate is fixed or any provider can send opus at different sampling rate?