Some apps are harder to read than others. If you have trouble making out, say, a location name in Google Maps, but you don't want to blow up the fonts across your entire phone, you can fix it app by app, or adjust the font size as you go.
First, make sure you have iOS 15 or higher via Settings > General > About. Then go to Settings > Control Center. Under More Controls, you'll see Text Size. Tap the green plus icon on the left, which moves it to the top under Included Controls.
Now, open the app in which you want to increase the font size. (We're using Google Maps as an example.) Swipe down from the upper right where you see the battery icon. Tap on the Text Size button (an icon of a small A next to a large A).
On the bottom, select whether you want to increase the font size on just Google Maps or across all apps. We selected just Google Maps. You can then increase the font size with the white bars. Close out of Control Center and your font adjustments should update automatically.
To increase the font size in any other apps, follow the same steps.