I'm Designing a mobile app (for android and iOS)
the "side menu" icon is on the left and when needed changes to "back".
when using the X button - should i position it the same as "back" - on the left or to put it on the right side for the screen.
i would suggest the "X" to be placed on the right side. as the action perform the least priority to close or leave the page and hope in your application it represent to close the page.
Better to align it on the right side of the screen.
Right Top corner.
In OS point of view more than 70% of world population are used to Windows OS on which it's to the top right corner and people usually search there.
You can place it on Right bottom corner(like few android browsers introduce URL bar at the bottom of screen) but that's not appropriate as accidentally user may press it.