How do shared content and local overrides actually work?

You set shared elements such as templates, components, navigation, catalog, policies, once at the core, and every property inherits them.

You set shared elements such as templates, components, navigation, catalog, policies, once at the core, and every property inherits them. When a site needs to differ, you override that element in place: the local version takes over for that property only, and everything else keeps inheriting. Change the core later and the update flows down to everything still inheriting, while your local overrides stay put.