The truth is a little less exciting, with Google inviting manufacturers to participate in legally binding agreements that ensured Google search sat atop everything in exchange for access to the Play Store, but the end result isn’t all that different. Microsoft could make its own phone without access to the Google Play Store, but that usually ends poorly for everyone involved. In order to have their apps and services installed on something running Google-powered Android, Microsoft needed to go the long way and offer compelling apps that users would want to install and use instead of the pre-loaded Google counterparts.
You may not be aware of it, but Microsoft is surprisingly close to making this a reality.
They are, indeed. There’s a reason Microsoft’s applications on Android are way, way better than those for Windows Phone or Windows 10: Microsoft Android is their plan B for when they do eventually end Windows Phone.