WiiM Mini supports AirPlay 2 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 2 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 2 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.