I think I can help you. It is important to understand the difference between updates and upgrades. An update is is when we install a newer version of the same operating system. It will be found in the System Preferences by clicking the Software Update button. If you already have the latest version installed, you will be notified that no update is available. An upgrade is when you install a newer operating system such as upgrading from Catalina to Big Sur. What you can install is dependent on the year and model Mac you have. Upgrades may be found in the Apple App Store. Or you can click on the links below. You should only use Safari for these downloads, never Firefox.I hope this information is helpful in clearing up any confusion.
Some upgrade links: High Sierra, Mojave, Catalina, or Big Sur.