Which layout is mostly used in Android?

The most commonly used layout classes that are found in Android SDK are: FrameLayout– It is the simplest of the Layout Managers that pins each child view within its frame. By default the position is the top-left corner, though the gravity attribute can be used to alter its locations.

Which is better constraint layout or RelativeLayout?

ConstraintLayout has flat view hierarchy unlike other layouts, so does a better performance than relative layout. Yes, this is the biggest advantage of Constraint Layout, the only single layout can handle your UI.