Now that is a complicated question. But usually the main way its accomplished with what we call antitrust laws. When enforced (and the usa has enforced thrm but not as strongly as it should) they effectively make monopolies illegal.
Of course the other key is keeping corruption in check and money out of politics, which isnlikewise a difficult task to do, but critical to a healthy capitalism
@icedquinn you're misunderstanding how American antitrust functions, as you can see throughout court filings, legislative proceedings, legal behaviors, and on and on.
The problem with American antitrust is that, in part because it is so broad, it runs up against other legal principles of the US government. The history of US antitrust in one of trying to reconcile paradoxes in the law that make it really unwieldy.
We have a government that on one hand is fundamentally bound to respect private property, but on the other hand, is charged with imposing on that same property.
We don't need any conspiracies from rich contractors to explain the issues with US antitrust law. The problem is with the US law itself being quite screwy, in its own right.