In what market does Amazon have a defacto monopoly in? Apple is another one often brought up for antitrust related to App Store policing.
In the late 90s, early 00s, Microsoft sold 90+% of desktop operating systems (their monopoly). That’s fine, great success! However, they used that monopoly to harm competitors: bundling a competing product with their earned monopoly (IE/Netscape), threatening vendors which attempted to also sell a competitor’s product (Sony/BeOS), locking in customers by extending common platforms and making them incompatible with competitor’s products (Java).
Had they not had a monopoly, those actions probably wouldn’t be considered abuse. It’s the combination that’s the problem.
MS had something around 97% market share with Windows alone. They used that position to release their free, and largely broken if you go by the standards, IE browser. By pushing that free browser, which cost them over a billion in marketing and development, they cut into the markets of the cross-platform browsers. This was their game plan. Web applications (even the rudimentary versions in the mid-90s) offered a chance for companies to leave Windows, to remove the need for dedicated applications on thick client-styled machines. The OS would be come irrelevant, and this is something MS feared.
But that wasn't the only product market that they did this with. They straight up bought competitors and either closed their product lines or sold them themselves. They threatened to penalize OEMs who sold non-Windows OSes by taking away their OEM license cost (much lower than the shelf price) for Windows, with the margins of that market that killed many alternate OS efforts. They just couldn't get into the market, why pay $50-200 for a new product when you already have Windows on your computer?