To customize Spree Commerce, you will need to create a new extension. An extension is a group of related models, views, and controllers that can be packaged together and used in a Spree application.
Once you have created your extension, you can add new functionality to Spree by overriding existing models, views, and controllers, or by adding new ones.
You can also use pre-existing extensions to modify the behavior of your Spree application. For example, the spree_shift commerce extension allows you to customize the look and feel of your Spree store.
If you need more help customizing Spree, you can check out the Spree Guides.