What is the difference between the hardware back button on Android VS the Navigation Back button and also the Navigation "Cross" button. Attaching examples. This is purely from a UX point of view and not on the functional side. From what I've heard, its basically going a step back VS Closing the whole process all-together. Is it really necessary to have a navigation back button?
The hardware button returns you to a previous state. If you switched between apps, it will take you back to the app you came from. Say for instance, you follow a deeplink from app A and it takes you somewhere in app B. The hardware buttons allows you to go back to app A, while any present back or close button in the app will take you to an other page within app B.
Also, it's also some sort of fail safe. There are ways to hide the soft keys on Android (Samsung still has them build into the phone). So with the correct navigation within your app, you're guaranteed your users will be able to navigate no matter.
If you're looking for a reason to remove those buttons, you could build your apps navigation on gestures (swipes). It has less affordance since it's not a visible form of navigation and is therefor not advised, but you could.