Your local database’s data is not pushed into Heroku, assuming you created Strapi with --quickstart that creates a SQLite (single-file based data storage) and in Heroku you would be using PostgreSQL (SQLite will not work on Heroku).
We do not currently provide tooling to sync the data over to the PG database from SQLite, see the following FAQ: Troubleshooting - Strapi Developer Documentation
Local modification should only be for testing purposes during code modification, in a case like yours all content will need to be created on the “production” Heroku instance