What happens when a new Activity is called? Is the memory from the old one released?

A new instance of Activity is created, and its lifecycle methods are called: onCreate(), onStart(), onResume().

Memory from the old Activity is not released immediately. It transitions to the Stopped state (onPause(), onStop()) and remains in memory until the system decides to unload it, if more space is needed for other components or if the user explicitly closes the application.