Do you have difficulty reading with some apps on your iPhone? iOS allows you to change the global font size in the various apps that use the dynamic font by simply going to Settings> Display and brightness, then select Text size.
You can further change the font size by going to Settings> Accessibility, then selecting Screen and Text Size and from there tapping Bigger Text for larger font options and dragging the slider to select the size you want.
Less well known is a small trick possible with iOS 15 that allows you to enlarge the font only in certain apps. Go to Settings> Control Center, scroll down until you find the “Text Size” item in the “Additional Controls” section and add the control in question to the Control Center.
From this moment on you can access the Control Center (by swiping from the top right at the bottom on iPhones with Face ID or from the other in old phones with Touch ID) and view the various options of the control center, including the new “Text Size” icon.
From the App of your interest (eg Facebook) go to the Control Center: from here you can tap on the “Text size” icon and change the font size only for that specific app.
Here is a video guide of how to go about it:
