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There is no action about creating a list item/file in the official documentation.
You could get all supported actions in the following documentation.
https://docs.microsoft.com/en-us/sharepoint/dev/declarative-customization/site-design-json-schema
Amos Wu
Microsoft SharePoint Community Support
Yup. This action currently is an experimental feature and hasn't been released yet.
Thanks for the information. Good to know that improving.
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