Code base architecture for software customisation needs?

I am looking for techniques, best practices, design patterns to design a code base architecture enabling me to share same code upon multiple projects.

Example : Facebook and Workplace by Facebook share the exact same social experience features except the fact that the second is more business oriented.

Challenge : How to deal with such a situation to be the most DRY possible knowing that this shared code include multiple layers (FRONT, UI, BACK, DATABASE ...)