Not a stupid question at all! It sounds like two things are going on here:
AppCenter isn't working properly. It should list a wide variety of apps, including LibreOffice and Firefox. See this questionthis question for a fix and this bug ticket for details on the underlying problem.
Ubuntu apps are often distributed outside the AppCenter in .deb packages. elementary OS doesn't come with a way to install .deb packages. This is by design, and in most cases installing an app from the AppCenter is simpler and more reliable than installing the same app from a .deb package. That said, you can install .deb packages by following these instructionsthese instructions.