The department joined 16 states and the District of Columbia to file a significant challenge to the reach and influence of Apple, arguing in an 88-page lawsuit that the company had violated antitrust laws with practices that were intended to keep customers reliant on their iPhones and less likely to switch to a competing device. The tech giant prevented other companies from offering applications that compete with Apple products like its digital wallet, which could diminish the value of the iPhone, and hurts consumers and smaller companies that compete with it, the government said.
Jason Snell made a good overview of what the suit focuses on. I think it makes the case easier to understand https://sixcolors.com/post/2024/03/u-s-versus-apple-a-first-reaction/