Android back button behaviour

Let's say we have a default, empty activity with default behaviour, launched with default intent flags. User presses back button on the device. The activity disappear... but how, actually?

• Is pressing back button behaving the same way like finish()?
• Is the activity immedietely destroyed (onDestroy is called)?
• Is the activity guaranteed to be destroyed, but not immedietely?
• Is there any chance the activity won't be destroyed and this instance will be reused when this activity is launched in the future? (so only onPause and onStop -> onStart and onResume are called?)

I'm looking for a reliable answer, so please do not answer if you are not absolutely sure what happens here.