You can connect the Pi, depending on version, either wirelessly or wired to your network.
The Pi will run Linux OS.
No receivers support Plexamp natively (ie built in)
This may help you if your new to Pis - Plexamp Pi With a Desktop - Setup Guide for Plexamp on the Raspberry Pi
I suggest you install Plexamp on your mobile first if you haven’t already, and get used to that as it will be used as the “controller” of Plexamp headless.
Edit: If your receiver support’s bluetooth then of course you could just bluetooth the audio from your phone running Plexamp to it, but if you want the best quality I would recommend using a Pi Audio HAT, eg Raspberry Pi Audio HATs | The Pi Hut
