How did business leaders work to increase their control over the economy using things like trusts, pools and holding companies?

Business leaders used trusts and monopolies (for example) to take over competing businesses.
With a trust, multiple companies worked together to eliminate other competition. With a monopoly, one business worked towards eliminating all competition.

Once competition has been eliminated, the trust/monopoly has enough power to regulate prices on that specific good/service.
This posed a danger to society, because there were no competing companies who sold the same service/goods for less money. This forced all customers to pay a high price because they could not find other businesses who offered it for cheaper.