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It's usually a button with a grid of dots ⋮⋮⋮ at the bottom center of your screen. Most of your files will be organized into folders. Most versions of Android have a stock File Manager app. If yours doesn't, go to the Google Play Store, search for File Manager and download one of the available free apps.
Open your device's Apps Tray. Tap File Manager. Tap a folder to browse the files inside it. Tap the magnifying glass icon to search for a file.