Powerapps make field visible based on another field

1. What is OnVisible in PowerApps?
2. How do you make a field readonly in PowerApps?

What is OnVisible in PowerApps?

OnVisible – The behavior of an app when the user navigates to a screen. Use this property to set up variables and preload data used by the screen. Use the App. OnStart property for set up once when the app is started.

How do you make a field readonly in PowerApps?

On the PowerApps form, select DataCardValue of the cards you want to have as View only, go to properties, Display Mode and change to "View". You can also find it on the left side options when selecting the field you want to change (See image).