Select path for storing file in Xamarin Forms

The System.Environment.SpecialFolder.Personal type maps to the path /data/data/[your.package.name]/files. This is a private directory to your application so you won't be able to see these files using a file browser unless it has root privileges.

So if you want the file to be found by users, you can not save the file in the Personal folder, but in another folder (such as Downloads):

string directory = Path.Combine(Android.OS.Environment.ExternalStorageDirectory.AbsolutePath, Android.OS.Environment.DirectoryDownloads);
string file = Path.Combine(directory, "yourfile.txt");

You must also add permissions to AndroidManifest.xml:

<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

Here is code to save an image for Android, iOS, and UWP:

Android:

public void SaveImage(string filepath)
{
    var imageData = System.IO.File.ReadAllBytes(filepath);
    var dir = Android.OS.Environment.GetExternalStoragePublicDirectory(
    Android.OS.Environment.DirectoryDcim);
    var pictures = dir.AbsolutePath;
    var filename = System.DateTime.Now.ToString("yyyyMMddHHmmssfff") + ".jpg";
    var newFilepath = System.IO.Path.Combine(pictures, filename);

    System.IO.File.WriteAllBytes(newFilepath, imageData);
    //mediascan adds the saved image into the gallery
    var mediaScanIntent = new Intent(Intent.ActionMediaScannerScanFile);
    mediaScanIntent.SetData(Android.Net.Uri.FromFile(new Java.IO.File(newFilepath)));
    Xamarin.Forms.Forms.Context.SendBroadcast(mediaScanIntent);
}

iOS:

public async void SaveImage(string filepath)
{
    // First, check to see if we have initially asked the user for permission 
    // to access their photo album.
    if (Photos.PHPhotoLibrary.AuthorizationStatus == 
        Photos.PHAuthorizationStatus.NotDetermined)
    {
        var status = 
            await Plugin.Permissions.CrossPermissions.Current.RequestPermissionsAsync(
                Plugin.Permissions.Abstractions.Permission.Photos);
    }
    
    if (Photos.PHPhotoLibrary.AuthorizationStatus == 
        Photos.PHAuthorizationStatus.Authorized)
    {
        // We have permission to access their photo album, 
        // so we can go ahead and save the image.
        var imageData = System.IO.File.ReadAllBytes(filepath);
        var myImage = new UIImage(NSData.FromArray(imageData));

        myImage.SaveToPhotosAlbum((image, error) =>
        {
            if (error != null)
                System.Diagnostics.Debug.WriteLine(error.ToString());
        });
    }
}

Note that for iOS, I am using the Plugin.Permissions nuget packet to request permission from the user.

UWP:

public async void SaveImage(string filepath)
{
    var imageData = System.IO.File.ReadAllBytes(filepath);
    var filename = System.DateTime.Now.ToString("yyyyMMddHHmmssfff") + ".jpg";
    
    if (Device.Idiom == TargetIdiom.Desktop)
    {
        var savePicker = new Windows.Storage.Pickers.FileSavePicker();
        savePicker.SuggestedStartLocation = 
            Windows.Storage.Pickers.PickerLocationId.PicturesLibrary;
        savePicker.SuggestedFileName = filename;
        savePicker.FileTypeChoices.Add("JPEG Image", new List<string>() { ".jpg" });

        var file = await savePicker.PickSaveFileAsync();

        if (file != null)
        {
            CachedFileManager.DeferUpdates(file);
            await FileIO.WriteBytesAsync(file, imageData);
            var status = await CachedFileManager.CompleteUpdatesAsync(file);

            if (status == Windows.Storage.Provider.FileUpdateStatus.Complete)
                System.Diagnostics.Debug.WriteLine("Saved successfully"));
        }
    }
    else
    {
        StorageFolder storageFolder = KnownFolders.SavedPictures;
        StorageFile sampleFile = await storageFolder.CreateFileAsync(
            filename + ".jpg", CreationCollisionOption.ReplaceExisting);
        await FileIO.WriteBytesAsync(sampleFile, imageData);
    }
}