https://arstechnica.com/gadgets/2022/03/microsoft-accidentally-reveals-that-it-is-testing-ads-in-windows-explorer/

Windows 11 testers are regularly finding new Windows 11 features that Microsoft wasn't ready to show anyone yet. Sometimes that means digging up a new Task Manager or tabs for the File Explorer. And sometimes it means finding advertisements for other Microsoft products as you browse your own locally stored files.

Microsoft MVP Florian Beaubois found an example of the latter when he saw an ad promoting Microsoft Editor while viewing his Documents folder in a Windows 11 build. In a statement to The Verge, Microsoft Senior Program Manager Brandon LeBlanc acknowledged that the banner ad was genuine, but he said that it had been "experimental" and that it "was not intended to be published externally and was turned off."