WiiM Mini supports AirPlay and legacy AirPlay receivers. Apple Music will be re-coded by iOS/macOS and transmitted to WiiM Mini in either lossy or lossless format decided by iOS/macOS and the music app.
The current AirPlay protocol doesn't support the high res audio. The audio is in the resolution of 16-bit, 44.1 kHz.
When you're using Music App on macOS, you can choose the "AirPlay icon" - 
or you can select the system-wide audio settings, then select sound and tap the AirPlay icon. We recommend you to choose the AirPlay from your Music App that will let you use latest AirPlay protocol. While starting from system-wide audio settings, it outputs every sound on your system to WiiM Mini (including your notification sound) and uses the legacy AirPlay protocol.