Corporate trusts were once a common method of establishing monopolies through stock trades. A handful of publicly traded companies compete in a given market. Key shareholders of the companies realize that they could make a lot more money if the companies cooperated instead of competed. So they place their stock into mutual trusts that consolidate ownership under a single umbrella. The trust gets voting control of the companies, with the shareholders becoming passive beneficiaries.

Standard Oil managed by John D. Rockefeller was one notable example. The idea being that there were a bunch of oil companies that could be forced together by their shareholders under a trust to adopt "standard" practice.

To translate it to this example, imagine if a marger between Paramount and Warner was not on the table at either company, but the majority of their shareholders decided to transfer control of their stock to a trust called "Media Satan". Media Satan gets a controlling influence in the business of both companies, starts aligning their interests to make as much money as possible, and the shareholders who placed their stock in trust get a share of the profit. All while the market is bled for all it's worth.

Antitrust laws generally targeted this practice. Break up a trust, and the companies managed by the trust go back to being independent.

The trust structure is largely no longer a thing, but not due to antitrust laws themselves. More that it's simply easier today to legally create a megacorporation with little oversight or barriers, eliminating the need for those sorts of loopholes. Antitrust laws are (supposedly) still able to be leveraged to break modern day corporations up if they get too big, but not as cut and dry when it's so much easier for a company to merge with or acquire another company outright.