Hey there dec_3_31_62,
Thanks for reaching out to the Apple Support Communities. We understand you have questions about the Double Click to Install when downloading apps from the App Store, and we'd be happy to provide you with the best information.
To get started, this would be expected behavior if you have Face ID enabled for the iTunes & App Store in Settings > Face ID & Passcode. If you disable this setting, you would be required to enter your Apple ID password whenever you download an app whether it's a paid app or free.
For more details, check out the following resource under the "In the iTunes Store, App Store, and Book Store in Apple Books" section: Use Face ID on your iPhone or iPad Pro - Apple Support
You could also use AssistiveTouch or Switch Control to confirm payments with Face ID instead of double-clicking the side button. More on that can be found in the following links below:
We hope that's able to help.
Have a great day!