Amazon, Google and Microsoft have spent the past year investing billions of dollars in artificial-intelligence startups—while also charging those fledgling companies a similar amount to use their cloud platforms.

The deals are making the big tech firms both the largest backers and most direct beneficiaries of these startups, reflecting how some of the AI boom’s biggest rewards keep going to the most powerful players. The value of the tech giants’ stakes could shoot up if the startups take off. And if not, they still will have turned chunks of cash into revenue.