Explanation:
Box 1: No
You can reorder screens.
In the left navigation bar, hover over a screen that you want to move up or down, select the ellipsis button that appears, and then select Move up or Move down.

Box 2: No
You can add screens to the app after it has been created.
Box 3: No
For example, you can change a button's shape. By default, Power Apps creates a rectangular Button control with rounded corners. You can make basic modifications to the shape of a Button control by setting its Height, Width, and Radius properties.
Reference:
https://docs.microsoft.com/en-us/powerapps/maker/canvas-apps/add-screen-context-variables
https://docs.microsoft.com/en-us/powerapps/maker/canvas-apps/controls/control-button