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

Designing A Better Back Button UX — Smashing Magazine

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Use the side, Home, and other buttons on your iPhone - Apple Support

Modal on the mobile: is bottom the best place for the close button? - User Experience Stack Exchange

What are these two buttons for?

navigation - UX idiom for closing activity: X-icon vs up-button vs back button - User Experience Stack Exchange

7 rules for mobile UI button design, by Justinmind

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dart - Flutter SearchDelegate BACK button doesn't execute close() - Stack Overflow

Back and Close button in the same native page - User Experience

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How To Enable The BACK Button On Your Android Phone Quickly!

What is the main difference between the cancel button and the back button in UX mobile design? - Quora

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