Change device‘s streaming direction

[Smigel]

.

Plan: using Iphone 12 Mini to control Raspotify on Raspberry Pi4 as a device 

Free/Premium: Premium

Country: Germany

Device

Iphone 12 Mini, IOS15.5 master), Raspberry Pi4,8GB RAM, RaspbianOS, actual update

My Question or Issue:

Iwant to use the Raspberry Pi as a sound-system which plays the music over a speaker.

I made a new app in the developer section and authenticated the Raspberry Pi‘s software Raspotify.

I can establish a connection between the named devices and controll respectivly the other one.

Unfortunatly the music can only be heared on the Iphone, but not on the Raspberry Pi. 

Does anyone have an idea, what to do, to hear the music on the Raspberry Pi?

Thanks a lot for your help!