Microsoft would have to be doing anticompetitive behavior to be monopolizing the desktop market. Things such as forcing OEMs to not allow linux support, or banning iTunes installation on Windows. Simply being popular isn't the same as a monopoly.
Apple has, on the other hand, been monopolizing their iOS devices with the app store. It is the only way to install apps on iOS and Apple has played fast and loose with competitors (like Spotify) in the past: https://www.macrumors.com/2019/06/03/eu-awaiting-apple-respo...