I'm trying to understand how cellular networks (4g & 5g) function and I've hit a roadblock in understanding OFDMA.

I understand that it's a scheme to neutralize interference on different frequency channels by placing them "orthogonally" on different frequencies, but what is totally confusing me is:

Where is this interference coming from ? As I understand it, we can send a radio wave on exactly one frequency if we want, without generating any other waves on other frequencies, I'm guessing this understanding is wrong, and that producing a wave on one target frequency will naturally produce waves on other frequencies in some predictable fashion, but I have no idea why or in what fashion.