The simplest radio to demonstrate and understand is a "Crystal Radio". A crystal radio has no amplifiers and only one tuned filter.

Fig. 1: Schematic of  1920s style Crystal Set with High Impedance headphones (moving iron or moving coil).
The 2000 Ohms earphone acts as resistor to set the capacitor discharge time constant.

Fig. 2: Schematic of  1930s style Crystal Set with High Impedance Piezoelectric or "crystal" earphone.
The ceramic earpiece is equivalent to a 2nF to 20nF capacitor (depending on Model)

Recently published on RTÉ's Saorview Website