When multiple strong signals with different frequencies enter a receiver at the same time, the inter modulation product at the receiver is generated by the signals by the front end due to the non-linear circuit of the receiver. Unfortunately, the frequency of the inter modulation product drops into the useful frequency band of the receiver and generates the inter modulation interference.
When a strong signal is reflected back from the transmitting end of a transmitter back into the transmitter, this signal, together with the transmitted signal of the transmitter may also generate some inter modulation products because of the non-linearity of the transmitter.
When multiple signals with different frequencies transverse across conductors at the same time, the inter modulation product is generated because of the non-linearity of the metals.
Because of the non-linearity of the receiver and the limitation of out of band suppression, multiple harmonic waves are generated on the received signal. When two strong interference signals are received at the same time, the frequency combination such as 2f1-f2 and 2f2-f1 of the two strong interference signals may drop into the band of the receiver and then generates interference.
The capability to resist inter modulation is a feature of the receiver. If you want to eliminate the inter modulation interference by installing a filter, you need to install a receive filter in the interfered system.