The .fileImporter modifier isn't for opening your SwiftUI app's documents. It's for opening other types of files in your app. The document struct's init that takes a ReadConfiguration is for opening your app's documents, either from a document picker (iOS) or by choosing File > Open (Mac).
Instead of using your document struct's init that takes a ReadConfiguration, write a function to process the JSON that takes a URL as an argument. Pass the URL the file importer gives you to the JSON processing function.
Topic:
App & System Services
SubTopic:
Core OS
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