The dispatch file in your charm is an executable shell script whose responsibility is to execute the src/charm.py file with certain environment variables.
The file is created automatically by charmcraft pack and you can inspect it by unzipping the .charm archive (unzip <charm name>.charm ) or by deploying the charm, SSHing into one its units, and inspecting the charm directory in there (e.g., for unit 0: ls agents/unit-<charm name>-0/charm).