To save GitHub private repository quotas, you might want to combine app and API repositories into one for monorepo management.

$ ls
android
api
ios

Using git subtree push, you can deploy only the api directory and below to Heroku.

git subtree push --prefix api/ heroku master

That’s all from the field about wanting to deploy only subdirectories to Heroku.

Reference Information

• How can I deploy/push only a subdirectory of my git repo to Heroku? - Stack Overflow

That’s all from the Gemba.