I dont understand what you mean by an example. But to put it simply, it is a derivative of the original signal. You are right that it is a multiple of the carrier frequency, but harmonic frequencies are very complex. The main concern with a transmitter’s harmonic emissions is the undesired signal outputs which are harmonically related to the fundamental signal rather than to other oscillator circuits. When interference from communications transmitters is experienced, it is a result of inter-modulation or defective equipment, which can interfere with frequencies within the same band.
I work for Ericsson as a Senior RF engineer and we see these kind of issues almost every day for cell phone towers., that use our equipment. Spatial Isolation in such situations usually helps a lot, unless it is not possible then you need to identify the source of the issue , which involves some times swapping the equipment or fixing a poor connection that was causing a high impedance mismatch.
A poorly built router (mostly the cheaper routers and repeaters) is where an issue like this is highly likely, but is relatively a rarer scenario.
A more common scenario will be channel over lap, which we all know and understand. Channel isolation is one of the ways to mitigate this kind of interference for sure, but it is not so easy because the channel that is ideal for WiFi in order to isolate from Zigbee, may not be the best channel for WiFi itself because a lot of your neighbor may be using it already. And unfortunately you cannot change your Zigbee channel . I’ve seen many times where the available WiFi channels are so limited as the neighboring WiFi networks are already using them.
In either case, having the hub closer to the WiFi router will cause an impact., as the WiFi signal is the strongest interferer closer to the router.