GPU
So that you can run interactive 3D applications like games in your browser.
battery
To adjust the performance of a web application to save more power.
Light/dark mode
To give you matching website.
active tab
Again to adjust performance or to pause an application.
OS
Is often used to give you a download link that matches your OS.
As a side question, why is the capability built in by a browser, but the user is never given a choice about whether or not any of this is shared?
You can disable or fake most of the stuff. But that usually makes you stand out even more in their statistics.