In previous versions of iOS, you could change the systemwide text size to make all apps—at least those that support Dynamic Type—display text at larger or smaller sizes. (Most people who use this feature want the text larger so it’s easier to read with aging eyes.) In iOS 15 and iPadOS 15, however, Apple lets you adjust the text size on a per-app basis, so you can increase it only for those apps where it really makes a difference for you. First, make sure Text Size is showing in Control Center by going to Settings > Control Center, and if it’s not in Included Controls, tap the green + button for it under More Controls. Then, while in an app where you want bigger text, invoke Control Center, tap the Text Size button, move the vertical slider to the desired setting, and then tap the App Only button so the setting affects only that app, not all apps.
This can be especially useful with some apps like Google and Google Authenticator, where you cannot see all the needed information with larger font sizes.