I think it’s the multiplication of the two waveforms (aka mixing). The spectrums are convolved. It’s a common technique. Square waves are a fun case to explo

I think it’s the multiplication of the two waveforms (aka mixing). The spectrums are convolved. It’s a common technique. Square waves are a fun case to explore here because you might start finding cases where something you might just consider 180 degree phase jumps (PSK) actually effect the magnitude spectrum (square wave). They don’t teach you that in school!