I understand that your screen goes dark after a few minutes of being idle, and I'm happy to provide some help for this.
Based on what you've described, it sounds like your iPhone is automatically locking. Check out the steps below to change how quickly this happens:
Passcode - iPhone User Guide
To adjust when iPhone automatically locks (and then requires the passcode to unlock), go to Settings > Display & Brightness > Auto-Lock.
Thanks for using the Apple Support Communities. Cheers!