How to integrate multiple "feature" react apps into monolithic PHP repo?
What approaches are there to manage multiple package.json files that live in each feature directory
  


One could leave the package.json in every feature directory and let the CI read the package.json when building the app. One could define the versions of dependencies in the package.json to get control about the app. If the app works with version A.B.C of a certain library then one could define this version to prevent that an update will break the app.


  
  How do we identify common packages and lock them into a root /package.json file. (Independently, all developers already use yarn)
  


Discuss this with the team and figure out what is the best solution for everybody. Multiple solutions are possible. Everybody should be happy with a certain approach.


  
  How to accomplish this with as little friction as possible for all react based developers - so that they get to simply continue with
  whatever they are already doing and DevOps handles the gritty bits?
  


DevOps in my opinion means that a certain team that consists of multiple specialties, e.g. test, dev, ops is responsible for a certain app. This means that the whole team should agree with a certain approach and everybody should work and handle issues according that process instead of DevOps handles the gritty bits which is about silos and "throw it over the wall"