The MK484 is a copy of the Ferranti ZN414. The ZN414 datasheet only shows a block diagram of the internals, but gives some idea of how it works.
It has several stages of rf amplification, AC coupled via small capacitors. The combination of small coupling capacitors and transistor cutoff frequency results in a relatively narrow bandwidth that covers the Long Wave, 455kHz IF, and AM broadcast bands.
These are followed by a transistor acting as an AM demodulator or 'detector'. Output current increases with higher signal strength, which increases voltage across the load resistor and reduces output voltage. This reduces bias current to the input stage which reduces its gain, providing some AGC (Automatic Gain Control) action.
The whole IC only has 10 transistors in it, so each stage may only consist of 1-3 transistors.