One frustration I used to have when using apps on the Apple Watch was that there wasn't a way to view notifications or access the Control Center from within an app. The only way to access either feature was to go back to the watch face.
The trick is to long press on the top or bottom of the touchscreen until a semi-transparent preview of either the Notification Center (at the top) or Control Center (at the bottom) shows up. To finish the task, swipe up or down.
You can then interact with notifications or toggle a device setting. When you exit, you'll be back in the app you were previously using.