Now that you mention the billions, I had a similar run-in with a client I worked for who meant to create a sticky-note tool for computers, which would allow you to sign up for a paid premium membership or grant you a basic tool.
What it does is that it allows you to post sticky notes on your desktop, within the browser or outside of it. It would allow for multiple users to link up and share their notes, to use for example in webdesign to leave notes for other programmers and designers on the page.
I worked for him a few weeks, provided social media posts, streamlined the process for him by creating a modular excel sheet where he could track all his social media activities across the board and much more.
But his payments were late and he demanded more and more work before he'd pay for already finished business.
I explained to him that technically, all copyrights remain with me for as long as he hasn't paid me yet (that's my basic understanding of how copyright and intellectual rights for freelancers work)
He somehow seemed to dislike the idea, which wouldn't be reasonable if he intended to pay me anyways, so he'd ultimately get the rights to everything.
I wrote an excessive ToS to state my rights and protect myself. Sent it to him. He didn't wanna sign it. So I froze use of my contents (which we handled through Google docs) and ended our little partnership.
He too told me that I'll regret it and that he'll surely make millions with it.
Well, if he does or did, more power to him. But future millions don't pay the bills that I've got in my postbox right now ;)