Like with many modern mobile apps, many features in the platform are available when you need them, or can be called upon. The search bar is hidden just off the top of the screen and can be accessed by swiping down on the screen slightly. You'll see the search bar appear just before a refresh of the page has initiated.
Simply type into the search field, and the platform will automatically surface matching post content as you type.
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If you swipe down to view content, then swipe back up. The search bar will be hidden again. Just swipe down and it will reappear.