Roon can actually be three separate parts. Beside the server and bridge (or end point), you can have a third device that does nothing but select and control the music to be played. This latter part is often on a smartphone or tablet device.
You can have all three parts on a single Windows or Apple Mac desktop or laptop, or you can divvy them up between two or three separate devices. The music can even be separate from the server, such as on a network drive, though both the server and the music drive must be on and able to see each other for things to work.
I ran a month long trial of Roon, running the Roon server software on my Linux desktop, with a Raspberry Pi endpoint and a Kindle Fire as a controller. (I ended up passing on the subscription. There were some things I liked about Roon but other aspects I didn't.)
