O que é Proof of work?

There is no shortcut to finding a valid hash; miners simply try enormous numbers of candidates. Verification, by contrast, takes a single hash operation, so any node can check a block instantly.

That asymmetry is the entire security model. Rewriting history requires redoing the accumulated work of every block since, faster than the rest of the network is adding to it. The security budget is therefore a real, measurable quantity denominated in energy and hardware.