A list of debug scenarios can be found on the main menu, double-clicking will start the game in a predefined scenario. Custom scenarios can be made by editing "DebugScenario.lua" at "ProjectZomboid\media\lua\client\DebugUIs\Scenarios", or creating a new scenario.
[ ] Add template for basic scenario file, with ample commentary
[ ] Add advanced template to test a series of dependent events or wait for user input before proceeding
[ ] Add function to generate a new scenario and return the file path (or open in a specified text editor)
From the Debug Mode wiki article: