How do I keep two apps open at the same time on my iPhone?
Keep an app open while checking a message (get a code without losing first app)
[Re-Titled by Moderator]
iPhone 7, iOS 15
You can move between open apps by double clicking the home button and selecting one of the apps cards in the App switcher. The one app should not close while checking the other app for a code.
click here ➜ Switch apps on your iPhone, iPad, or iPod touch - Apple Support