Craft 4 uses modern
.env style configuration, learn more about the concepts here. This means you can run your Craft locally and remotely without code or configuration file changes. Locally, your
.env file will be modified and read. That file is not part of Git, you don't need it on the App.
No need to configure the MySQL database connection for fortrabbit. On fortrabbit the environment variables are seeded from the ones set in the fortrabbit Dashboard (not from the .env file). If you chose Craft in the Software Preset when creating the App, all ENV vars at fortrabbit will already be pre-populated. If not, see here.