They started appearing in their embedded applications Windows 10 the rounded corners of the windows. See the calculator and Photos.
The new look of Windows 10 comes in the built-in operating system applications as the company has started working on a new rounded corner in the utility windows.
Applications such as Calculator and Photos have already taken on a new shape in the utility windows that appear after a user right-click.
The first changes that are logically visible on your computers, come in the appearance of the programs Calculator, Photos, Movies and TV, Maps, Calendar and Sticky notes.
This is most likely due to the application of the new WinUI controls in the applications. So far the change has not penetrated into their very shell Windows, but from the screenshots posted by Microsoft, it really is a matter of time to see the rounded corners everywhere.