Why Interference can come from either end of the link. Fundamental overload happens when your strong on-channel signal simply swamps the front end of a nearby consumer device, even though your transmitter is clean. Harmonics are unwanted multiples of your operating frequency (2f, 3f, and so on) that can land in TV, VHF, or other bands. Spurious emissions is the broader category of any unintended output outside the necessary bandwidth, including mixer products and parasitic oscillations, and Part 97.307 requires transmitters to keep these suppressed.
Watch out Picking just harmonics is tempting because they are the classic cause, but that ignores overload, where the interference is really a weakness in the victim device's ability to reject a strong nearby signal.
Interference comes from a dirty transmitter (harmonics, spurs) or an overloaded receiver, so all of the above.