Suggest you read the top of the Memes
bread >>482965 where the social media guidelines and redpill tactics are posted. Someone went to some trouble to collect that info for memers who don't have social media accounts themselves.
You are up on classical physics but I think you would benefit from reading on Non-Linear Optics. It is a weird branch of physics and relevant. Here's a taste.
Basically the mixing can occur in any non-linear medium. A plasma can be such a medium. A colloid can also do it. The human body actually can act as a non-linear medium. Matter in a strong magnetic field can also do it if I recall correctly.
Mixing of signals is not exclusive to vacuum tubes and semiconductors although they are fruitful for studying the phenomenon in classical physics and RF engineering.