This has to be really early days for the UI; the phone UI heritage screams at you. For example: There’s no clear and evident “shutdown” or “reboot” interface.
If you click on your “User tile” on the start screen/menu/page you get an option to log off.
Sometimes, right-clicking screens works and you get an app-wide context menu. But it doesn’t seem consistent.
The lack of a close-app button is bothering me greatly, hehe.