if you mixing yarn and and npm create a separate directory e.g. mobile and move app.json eas.json and other files there. There should be no pacakge.json in the root. This setup would not be called a monorepo having it this way is just separate project in one git repository.
other option would be to just use one package manager which I highly recommend (but in this case I also would move everything related to mobile app to the subdirectory)
In that case, the index.js file (that registers the entry point) which, according to the documentation, has to be at the root of the project, should be moved to the app.js right?
What about the .easignore file in monorepos? I have read here that it should be placed in the root folder of the project. What root is that? The root of the monorepo or the root app sub-project?
Also, eas build has to be run from the packages/app/ folder right?