I don’t won’t to step on the toes of others that have already answered some of your second post, but I note a few things that may still require clarity.
You initially mentioned content on external hard drive and if that’s all you wish to do then the PC will not be necessary to play that content.
The same can be said for Tidal and Qobuz.
But if you also want to employ Roon then (I believe) a PC or other equipment will be required, but I’m really outside my experience with Roon so it’s best others expand on that aspect.
Regarding the functionality of the Aries being the same as the PC, simplistically “yes”, but with reservations. If you watched the video one major aspect talked about is reducing “electrical noise” which PC’s leave something to be desired in that area. The Aries has been optimised on that front (as are other streamers) so consequently they can sound a lot better.
I have the Altair G1 that basically combines the Aries and Vega aspects into one box. It solely works over WiFi with content stored on ssd fitted inside (it could also be attached to the external USB port) and I navigate via LDS app on iPad Pad.
That’s it, I can listen to my music, radio streams and if I had the accounts for Tidal or Qobuz, those as well all from that simple setup.
The only time the iMac is used (as already explained) is to ‘manage” that content, for example add more music to the drive or edit it’s metadata.
Hope this gives a little more detail.