In a nutshell: Google has been working to bring a "proper" desktop mode to Android for quite some time. The feature was first introduced in 2019 with Android 10, but it wasn't intended for end-users.
Android 15 QPR 1 Beta 2 has added desktop windowing to the Pixel Tablet, preparing for an eventual rollout to the Android platform in the future. Desktop windowing allows users to run multiple apps ...