I think in most cases you’ll put HQP on the machine with the Roon core, but there’s some flexibility here.
When Roon is playing through HQP, it inserts a special “Roon” item in HQP’s playlist, and then passes a bit-perfect stream to HQP via that playlist item. HQPlayer owns the relationship with any playback hardware. You might be using HQPlayer to play to a local/usb output, or you might use NAA to move the audio stream elsewhere.
When configuring HQPlayer in Roon, you’ll type the IP/network address where HQPlayer is running into Roon, so there’s no requirement that they be on the same machine.