internet's physical medium, for example: copper-wire cable can be cut & connected to a device that is put in between both parts of now-cut-in-two a cable.
this device (C) can monitor & control traffic between devices (computers for example) A & B, and can have additional the Internet connection for receiving remote commands.
this device can be called 'relay' or 'router', 'switch', 'network eavesdropping device' or simply: 'a bug'.
if for example electromagnetic field can be observed to extract electric signal, then this device can be called 'transducer' and would not require cutting a cable.
that's how passwords can be sniffed for example, in preparation for more hacks, for example.
there's risk of leaving fingerprints, so gloves can be used.
there are ethernet splitters & wireless access points.
wireless access point can be in sync with hidden computer, that can store data, transmit data via network or configure access point as neccessary & needed.