How is the internal circuitry of a microcontroller designed?

To put it bluntly, it's also an ordinary circuit, the principle is the same, just using micro-technology, such as laser etching and other ways.

A long time ago, the integrated circuits, the volume is relatively large, just like the current circuit board, you package it up, it is equal to an integrated circuit.

With nanotechnology, the size will get smaller and smaller, but the principle is the same, design the circuit, produce the core, and then package it.