Use Screen Time to lock apps with a passcode
Open Settings
Tap Screen Time
Tap Turn On Screen Time (if needed)
Tap Use Screen Time Passcode
Set a Screen Time passcode
Tap App Limits
Tap Add Limit
Select the app(s) you want to limit
Set the time limit to 0 hours and 1 minute
Tap Add
To lock access, wait until the limit is reached
To prevent changing limits, enable Screen Time passcode and do not share it
Use Guided Access to lock to one app
Open Settings
Tap Accessibility
Tap Guided Access
Turn on Guided Access
Open the app you want to lock
Triple-click the Side button (or Home button on older models)
Adjust settings as needed
Tap Start
To exit, triple-click again and enter the Guided Access passcode