This boilerplate provides an example of a React / Django (Client / Server) based application hosted on Heroku with a Gunicorn Django setup.
The .buildpacks
file is used to use multiple Heroku buildpacks for different sub directories.
Find more about that file in the Buildpack documentation.
The app.json file is there for specifiying all the dependencies of your Heroku application. These can be addon-specific ones (Postgres e.g.), environment variables which needs to be set or hardware specification for you dynamic created environments via Review Apps.
Find more about that file in the Heroku docs.
Via this file you instruct Heroku which processes should be run on the Dynos (containers). Also you can set there a command which will be execute on each release to run e.g. SQL migrations.
Find more about that file in the Heroku docs.