From: Require a password for App Store and iTunes purchases - Apple Support
Require a password on your iPhone or iPad
1. Open the Settings app.
2. Tap your name.
3. Tap Media & Purchases.
4. Tap Password Settings, then choose the setting that you want.
About turning off password protection
- You can't completely turn off password protection for paid items on your iPhone, iPad, or computer. On Apple TV, you can turn off password protection for both free and paid items.
- If you enabled Touch ID or Face ID for App Store and iTunes Store purchases, you'll be asked to use Touch ID or Face ID for every download, even if the item is free.