There are a few variants on how to do a closed loop system. OpenAPS is phone agnostic and everything runs on a RaspberryPi.
I myself use Loop. It's an iOS app that uses a RF-to-BT bridge to talk with the pump and intercepts the CGM reading from the official Dexcom app (there are forks that support different methods, being the Dexcom source the "official" one). The bridge is the size of a 9V battery, I never stopped to see which processor uses but it's probably 8 bit.
Each system support different pumps, sometimes people want to use one but are stuck with a different one to be able to use their pumps.
I myself use Loop. It's an iOS app that uses a RF-to-BT bridge to talk with the pump and intercepts the CGM reading from the official Dexcom app (there are forks that support different methods, being the Dexcom source the "official" one). The bridge is the size of a 9V battery, I never stopped to see which processor uses but it's probably 8 bit.
Each system support different pumps, sometimes people want to use one but are stuck with a different one to be able to use their pumps.