Both the Mac mini and the iPhone are Bluetooth compatible. But the features they employ are not such that audio can be transferred in the way you are looking for. Certain file transfers can be made; photos, AirPlay video streaming, AirDrop, iCloud file sharing, etc.
The mini can output audio to Bluetooth headphones for music, and can input audio for, I believe, Siri commands and voice navigation. Likewise the iPhone.
The spec is not used for direct audio interconnect between two devices, however.
It may be possible to hardwire connect the two with the use of additional audio interface hardware and software that I am unfamiliar with, but direct from mini to iPhone with standard macOS and iOS apps is a non-starter, afaik.