How I turned my old Stereo Amp To A Smart Spotify and AirPlay Player

Smart everything is the new Standard for all kinds of devices, even in the good old HiFi amplifier segment. New amps have fancy connectivity like WiFi, Soptify and AirPlay support built-in. But why buy a whole new Amp if your good old HiFi stereo still works just fine? In this project I want to use a Raspberry Pi as a smart Audio source that can stream Spotify and audio from iOS devices via AirPlay.