Backoffice also comes with two Extension types which can be used to register more extensions.
Using bundle to declare other Extension Manifest
[TBD link to article on the matter].
Using entryPoint as your foundation
This can be used as an entry point for a package.
The entryPoint extension is also the way to go if you want to load in external libraries such as jQuery, Angular, React, etc. You can use the entryPoint to load in the external libraries to be shared by all your extensions. Loading global CSS files can also be used in the entryPoint extension.
Read more about the entryPoint extension type in the Entry Point article.