If you take a quick look at the Camera app, you will notice the little gear icon in the top left corner; that's for Settings.
I suggest maximizing your Camera app then clicking the Settings icon - otherwise, it simply doesn't show up.
At the top of the bar on the left you should see "Pro mode." Make sure that is "On" and then go back to the full Camera screen.
On the left side of your screen, below the timer, you will now see a small icon. That is now the Brightness and you can adjust it.
You are absolutely correct that there should be a full control panel to adjust brightness, contrast, etc. but there just isn't.
Best of luck.
This is actually very useful! It is in the built in camera app. In which, you can actually switch between the builtin camera and the webcam.
There is a button at the top right corner to do the switch (very similar to the button you use to switch between front and rear cameras in your phone).