How to read files created by the app by iOS WidgetKit?

You should use App Groups Capability to share data between your targets.

Here is a good tutorial by RayWanderlich

In order to read files created by the iOS widgetKit, you need to create files in the shared container

let url = FileManager.default.containerURL(forSecurityApplicationGroupIdentifier: "yourapp.contents")?.appendingPathComponent("hello")
let data = Data("test read".utf8)
try! data.write(to: url!)

And you can read the data in the Widget class

@main
struct StuffManagerWidget: Widget {
    let kind: String = "TestWidget"

    var body: some WidgetConfiguration {
        IntentConfiguration(kind: kind, intent: TestIntent.self, provider: Provider()){ entry in
            WidgetEntryView(entry: entry, string: string)
        }
        .configurationDisplayName("My Widget")
        .description("This is an example widget.")
    }
    
    var string: String = {
        let url = FileManager.default.containerURL(forSecurityApplicationGroupIdentifier: "yourapp.contents")?.appendingPathComponent("hello")
        let data = try! Data(contentsOf: url!)
        let string = String(data: data, encoding: .utf8)!
        return string
    }()
}