Mendix does not actually generate code. Instead, the model is interpreted at runtime.
The code that interprets the model is proprietary/closed source, so you can't actually see any of the magic.
However, you can export the model in a structured format through the model SDK.
As Romain pointed out, appstore components are (often) open source and you can inspect their source code through github or by exploring the mendix project folder locally.
You could certainly find some of them here : https://github.com/mendix