To hide an app, you'd want to press on an app icon, then choose Require Face ID > Hide and Require Face ID.
The app will no longer show up on your iPhone, won't appear as recently used, and can't be searched using Spotlight on your iPhone.
To lock an app, you'd just find it on the Home Screen, touch and hold the icon, and then choose Require Face ID > Require Face ID.
Specifically, you'll find it under Battery Usage By App, which is the big list of apps that have used iPhone battery recently.
They'd need to know your iPhone passcode to get in – or pass a Face ID or Touch ID check, which wouldn't work.
