The iPhone camera app has a touch system for selective focus and exposure. Open the camera app and touch where you would like the exposure taken. It focuses also but more on that later.
I can't replicate the box that appears but the X will have to do. As this demonstrates, touching the tree in full sun will make the ground too dark.
Touching the ground in full shade makes the trees way too bright.
Experiment by touching other areas. I usually can find a spot that balances the light better.
Remember that the iPhone app will focus at the same point as you set exposure so give some thought to that. The focusing is much more forgiving so this, fortunately, is not that hard.
The creator of this guide has not included tools