One you have cloned the repository, make sure you have installed and you are using the ruby version 3.0.1 (with ruby -v).
Now, run gem install bundler and then bundler to install the gems of the projects. Later run yarn to install yarn dependencies.
Then install the Heroku Command Line Interface (CLI) with brew tap heroku/brew && brew install heroku. If don't use Mac, you can check here the instructions.
Don't forget to create the database running rails db:create and rails db:seed, otherwise the Rails app will not work.
After that, to start the core-backend in the development mode, you can run:
It will be listening on http://localhost:3000.