This is my simple way to lock apps on my iPhone using the Shortcuts app. I walk through how I set up a shortcut that locks the screen when a specific app is opened. I picked Facebook in this example,

There is, however, a way to share your iPhone with someone and control what they do with the device without peering over their shoulder. The answer is Guided Access, an accessibility feature that allows you to lock others to a single app and restrict the number of available features.