Hi Marc, welcome to our Roon community!
In the Roon ecosystem, the RPi is most often used as a bridge device. This is, you already have a Roon core device connected to your network, best by Ethernet cable. Then you can put a bridge device anywhere you like, as long as you are able to connect it to the same network your core device is connected to. Again, Ethernet is preferred. The output from the bridge device gets connected to a digital input of your DAC, and from there analog output to your preamp/amp.
The RPi per se can be directly connected to your DAC using one of its USB ports, but if you prefer to connect it via S/PDIF (coax or optical), extension cards (“hats”) are available for the RPi from various specialized hardware manufacturers.
The RPi is generally being set up with a specialized Linux version as operating system, and this operating system is burned onto a memory card which can be installed directly into the Raspberry Pi.
To operate as a Roon bridge device, the Roon Bridge software package must be installed into the operating environment of the Raspberry Pi computer. The most simple and elegant form to do this is using the excellent RoPieee distribution, which is just a specially tailored Linux distribution with the Roon Bridge software preinstalled. You only have to download the RoPieee package, ‘burn’ it onto the memory card, install it into the RPi and power up. If your Pi is on the same network as your Roon Core device, the core will detect the bridge and you will be able to configure in Roon to output on your RPi/Bridge.
To burn the RoPieee package onto your memory card, you can use free downloadable application like Etcher.
So, for the simplest setup you’d need this:
Raspberry Pi computer; recommended: RPi 4B, 2 GB RAM
A case to put your RPi into; the RPi 4 can get warm or hot, and a passively cooling case is recommended. I like very much the Flirc case which works well keeping your Pi cool. Please note that if you prefer to connect your Raspberry Pi by other means than USB, than you need an extension (‘hat’) card, and this Flirc case won’t accommodate this extension card,
A power supply for the RPi. This can be the original power supply or an equivalent like the one offered by Canakit.
A micro-SD memory card; 16 GB are sufficient.
With these items you can assemble yourself in 15 minutes a working Roon Bridge device to be connected via USB to your DAC.
I hope this is somewhat helpful to you; if you have more specific questions, just ask. You’ll get help from someone here on this forum.