How do I know when to use back or close button (or both) on mobile

I want to know when and why should I use the X or back button on top of the page. And additionally, related to that, how should I judge the transitions, when should a page just slide from the right

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How do I know when to use back or close button (or both) on mobile

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How do I know when to use back or close button (or both) on mobile