Lock the app
Open it with Face ID. Some things it cannot hide, so those are written down here too.
Set it up from App lock under Privacy in Settings. It is a Pro feature.
With it on, opening the app asks for Face ID (Touch ID on some models, or your device passcode).
Let quick trips through
Being asked every time gets tiresome, so Lock after lets you choose a delay — Immediately, or a few minutes. If you tend to close the app and come straight back, leaving a gap is easier.
What it cannot hide
The app lock asks when you open the app. It cannot hide the text shown in notifications, or saving from the share sheet.
If part of the body showing in a notification bothers you, hide previews in the iPhone’s own notification settings.
Note that this feature is unavailable if your device has no passcode set.