Any auto made in the last 20 years is incredibly complex. The biggest advancement has been hiding all that complexity.
I have swapped engines and welded cars together, so this is coming from experience. Every car has many sensors, sometimes more than 100. All these sensors feed into enormously complex software state machines written to do all calculations on a RTOS in bounded time and in a verifiably correct manner. This complexity started in, but doesn't stop at, the engine.
You are vastly underestimating the effort needed to duplicate a modern car. There have been times where I've sat for several minutes just marveling at the complexity and beauty of small parts I can hold in my hand. Some of them were surely a years work for an engineer. On any car there are thousands of these clever little parts. For roughly 20k you're buying a product of many thousands of years worth of work.