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When you install Revit correctly, there should be a folder for "Imperial Library" or "Metric Library". That's where all the basic families are located. If you download families from other sources, then put them in the library. Or put them in a custom folder, and then use Insert > Load familiy, and go to that folder and load. The families don't always appear on the screen. Sometimes they just load silently, and you need to find them in the Project Browser to insert them.