This is something I keep meaning to find out about. It's probably a silly question to many, so sorry in advance.
Let's say I have a PC and a NAS. If I have them connected directly to a Router, obviously any traffic sent between the two of them is going via the Router. If I connect a Switch to the Router and plug both the PC and the NAS into that Switch, does all the traffic still get sent via the Router?
It depends. Traffic won't leave the switch if it's doesn't have to (e.g., traffic directly between the PC and NAS). But if either need to access some device on the router's switch, or the Internet, then yes, they can and will use the router's switch.