How to use Photos extensions
> Extensions in Photos allows you to access many of the editing features in apps such as Pixelmator, Affinity Photo, and Luminar without leaving the Photos app. There’s even a category on the App Store that lists the apps that have Photos extensions.
To see that, double-click any photo then choose Edit from the top-right corner of the window. Click on the More button (…) and choose App Store. In that menu you’ll also see an option called Manage. Click that and you’ll be taken to the Extensions pane in System
Preferences, from where you can enable extensions for those apps that support them. Once enabled, the apps appear in the Edit menu.
Some apps have only one option, others have several. The difference is that, say, Pixelmator offers one option that takes you to a ‘Pixelmator view’ inside Photos that provides access to Pixelmator’s editing features. Affinity Photo, on the other hand, displays multiple options in the Edit menu, each of which takes you directly to a specific editing tool.