When I connect to "Ethernet" (Ethernet is the Ethernet port at my PC) at Wireshark, then i can see comms between PC and Dev1 or PC and Dev2. But how do I see comms between Dev1 and Dev2?
With a simple, unmanaged switch tapping into the conversation of two other devices isn't possible.
You either need to capture directly on one of the devices or, when that isn't possible, you need a managed switch with port monitoring, port mirroring, or SPAN capability. On such a switch, you'd set the monitor port to the one your Wireshark PC is connected to, and mirror the traffic from one of the other ports.
Depending on the switch, the monitor port may become transmit only/dedicated for capturing, so the Wireshark PC effectively becomes detached from the network. You may need a dedicated NIC to work around that.