Password-Protection for Apps
Niagara Launcher does not provide a way to lock apps, because non-system launchers cannot hide and lock apps entirely (without root permissions), e.g. they still appear in Android's settings and you can launch them with another launcher. A password lock option adds only a pseudo layer of security on third party launchers.
I want to hand my phone to a friend but they should only have access to one app, what should I do?
There's an Android feature that exactly does that, please take a look at https://support.google.com/android/answer/9455138