Steps

• Deploy DB to Atlas

• Add MONGODB_URI in .env file

• Heroku create new app either on dashboard or via CLI

• Add .env variables in settings >>> config vars

• Netlify import repository

• In Netlify's build command fill out this: cd frontend && npm i && npm run build

• In Netlify's publish directory fill out this frontend/build

• Add frontend .env variables set CI to false

• Add REACT_APP_SERVER_URL to https://yourHerokuAppName.herokuapp.com/api

• Change Netlify's domain.

• Add clientURL to config vars in Heroku