Launch the Mac App Store and if not already, sign-in with your Apple ID. In the Mac App Store search window, ente Extensions. That will find many, but not all Safari extensions, and if you cannot find it as an extension, then search for it by name. A good example of this is the Grammarly for Safari product which installs the Grammarly extension (but does not work in these posting areas).
Typically, you do not directly install an extension, but rather each of the Safari Extensions shown in the Mac App Store is a downloadable application, that when run will install the Safari extension for you. Enabling that extension and keeping it current are up to you in Safari Preferences.