How, when foundries are a natural monopoly? How can you realistically create a startup to build a multibillion / multi year fab ?

There are 3 foundries capable: Samsung, Intel, and TSMC. Companies can contract out some to Samsung and Intel. TSMC can (and is) build more fabs.

I don't think that laying bricks and operating a crane is on the same level(or universe) of complexity as making and operating high-end semiconductor machines.

Ideally. Companies may also be hesitant to invest in expanding production capacity if they assume this is a bubble though.

Companies are always slow to expand production capacity. It takes time to react to unforeseen shifts in the market - but once it does happen, the scale can be quite huge.

(Who's gonna tell him)

The free market as described is imaginary. Large companies due to their size hold a lot of power, and can use a mix of regulatory capture and monopolies to make better ideas financially infeasible to bring to light.