In the Builder/src/config there is another folder builder.
Inside you will see
apps.json - The templates that the client will see
components.json - All the components that are available
sections_config.js - Configuration for each section, their display name, an icon.
appsettings.json - JSON settings that will be default per each new app.
You are able to modify all of them or add new temples and components.
Save the files. You will need to build and republish the app builder
When your client, creates a new item. ex. Event in the Event Section.
Initial data will be used to create this event/item. And he will be able to modify the newly created event.
You have the option to modify this initial data that will appear to the user. Or you can make it empty.
All of the sections initial data is located in Builder/src/firestoreschema.js.
Ex. Here is how to modify event dates
Save the file. You will need to build and republish the app builder