Fruits are wet, salty, acidic objects. As such, they can be used in an electronic circuit. They have an impedance, and, if you do it right, a voltage. [1] There's nothing fake or surprising about this.
He's just placing the fruit in series with the power source of some effect pedal. Unsurprisingly, this slightly alters the power source for the pedal, creating an audible difference in effect.
It's probably not something someone would do in practice, but notably, when it comes to electric guitars, experimenting with different types of distortion is pretty common, and it's common for guitar players to get creative.
Even if this "fruit effect" is not practical (I mean, you'd get inconsistent results), it's definitely a silly and interesting enough idea to justify a YouTube video. I could even see someone using it as a gimmick for playing live.
> I think anyone with a basic understanding of electronics would see the absurdity of these circuits and wouldn’t buy the schematics
Which schematics are you referring to? I think you might be assuming he's selling the "fruit" circuit. I think he's selling circuits for effect pedals, completely independently of this "fruit" video.