It’s not quite direct because of how the ethernet protocol works.
First Roon will want to send a packet to the Aries. The kernel on the machine (doesnt matter the platform) will send an ARP-Who-Has out it’s ethernet port. The ethernet switch it is connected to will see this packet and broadcast it to every port on the switch other than the one that sent it. This packet contains a query that say “if you have or know of this ip, please tell me the MAC address where I should send the packet to”
In your case, the Aries will reply with an ARP-Is-At packet saying “I am IP X, and at MAC Y”. That ARP response is also broadcast to every port on the switch and anyone listening there will take note that X is at Y (in what is called the ARP cache).
Now, whenever anyone needs to talk to IP X, they will send out the packet to MAC Y, which the switch will know is on some port. Let’s say the NUC is plugged into port 1 on the SG105 and the Aries is on port 2, and the Archer is on port 3. Because the switch got the packet from port 1 and it is destined for a MAC that it knows is on port 2 (because of the ARP-Is-At), it knows to send it ONLY to that one port. Port 3 (the Archer) will never see it.
There is something called an ethernet “hub” that differs from a ethernet “switch” in that it always broadcasts all packets to all ports. But those devices don’t really exist anymore and they cant sustain high speeds. On these old hubs, a maximum of 10mbps, maybe 100mbps is reachable. To do 1000mbps, you need a “switch”. Sometimes these are referred to as dumb hubs and smart switches, but that’s confusing because there is also “managed” switches which generally have more advanced features, usually just causing trouble unless you really really know what you are doing. Managed switches sometimes are referred to as “smart” as well. I know, annoying.
As for your ethernet cable: If you believe that the ethernet cables matter (not everyone does), it has to do with EMI/RFI noise interfering with your DAC (or something else) and not the digital bits being transferred. In which case, all the cables and all other things matter, whether they are connected (EMI) or even if they are nearby (RFI).
Before anyone tries to argue the “bits are bits” topic with me, please read the post linked below and the one I made 2 after it:
https://community.roonlabs.com/t/validity-of-audiophile-ethernet-cables/30251/22