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First, swipe up from the bottom of the screen and this will bring Control Center. Make sure the rotation lock is not enabled there (if it is enabled, you will see a lock inside a curved arrow)
Then open Camera. Then hold the phone in horizontal position. You will see the icons rotate matching the position you hold it. Then press the record button and this will record the video in landscape/horizontally or 16:9 mode. DO NOT start recording the video before holding the phone in horizontal position.
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