Open the Shortcuts app
Tap Automation
Tap Create Personal Automation
Choose App
Select the app or apps you want to lock
Tap Is Opened
Tap Next
Tap Add Action
Search for and select Lock Screen
Tap Next
Turn off Ask Before Running if available
Tap Done
Use Face ID, Touch ID, or your device passcode when the app opens
For built-in app limits, open Settings
Tap Screen Time
Tap App Limits
Tap Add Limit
Select the apps
Set a time limit
Tap Add
Set a Screen Time passcode when prompted
Use the passcode to extend app access