The way that you do it matters. Participating as equals in the marketplace you've created is fine. Leveraging data or other things your competitors don't have access to, is abuse as no one would be able to compete against you in that case.

Amazon using data from its own marketplace to know what "Amazon Basics" to produce is one example, Apple being able to use private APIs other applications in the App Store cannot use is another.

Yup. There are laws on the books though like the Sherman Act, Clayton Act and Robinson-Patman Act that were passed in response to firms monopolizing industries and coercing competitors.

If firm gets to powerful they can do anticompetitive things that make the American economic system run by powerful oligarchs instead of a more democratic free market.

Capital tends to concentrate so regulation is needed to keep the free market system intact. In other words an economy run by monopolies isn’t a free market.