iOSのファイルアプリ（UIDocumentPickerViewController）を開いてドキュメントフォルダに保存したファイルを開く

ContentView.swift

import SwiftUI

struct ContentView: View {
    @State private var fileUrl: URL?
    @State private var showingPicker = false
    
    var body: some View {
        VStack {
            Button {
                let documentDirectoryUrl = FileManager.default.urls( for: .documentDirectory, in: .userDomainMask ).first!
                let fileUrl = documentDirectoryUrl.appendingPathComponent("snorlax.txt")
                try! "I Love Snorlax!".data(using: .utf8)!.write(to: fileUrl, options: .atomic)
            } label: {
                Text("Save File to Document")
            }
            
            Button {
                showingPicker = true
            } label: {
                Text("Show File Picker")
            }
            
            Text("FileUrl: \(fileUrl?.description ?? "nil")")
        }
        .sheet(isPresented: $showingPicker) {
            DocumentPickerView(fileUrl: $fileUrl)
        }
    }
}

#Preview {
    ContentView()
}

DocumentPickerView.swift

import SwiftUI
import UniformTypeIdentifiers

struct DocumentPickerView : UIViewControllerRepresentable {
    @Binding var fileUrl: URL?
    
    class Coordinator: NSObject, UIDocumentPickerDelegate {
        var parent: DocumentPickerView
        
        init(_ parent: DocumentPickerView) {
            self.parent = parent
        }
        
        func documentPicker(_ controller: UIDocumentPickerViewController, didPickDocumentAt url: URL) {
            self.parent.fileUrl = url
        }
    }
    
    func makeUIViewController(context: Context) -> UIDocumentPickerViewController {
        let documentPickerViewController =  UIDocumentPickerViewController(forOpeningContentTypes: [UTType.text])
        documentPickerViewController.delegate = context.coordinator
        return documentPickerViewController
    }
    
    func updateUIViewController(_ uiViewController: UIDocumentPickerViewController, context: Context) {}
    
    func makeCoordinator() -> Coordinator {
        Coordinator(self)
    }
}