What is the best way to work with differente versions in a ReactJS project?

I'm working in a React project and basically we have a core project, with a set of functionalities, and from this core project we have customised projects with the functionalities of the core and more functionalities specific according to each the customisation. And sometimes will be necessary to include a new functionality in the core and that functionality will replicated to the custom projects.

I would like to know what is the best way to handle this kind of situation in React, how to control the projects (core and custom) and keep them in sync? Is there some pattern in React for this kind of situation?