使用此命令获取目录：

Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS);

不要忘记在manifest.xml中设置此权限：

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

现在从你的问题编辑我想更好地理解你想要什么。文件显示在下载菜单上从一个红色圆圈是那些实际上是通过DownloadManager下载的，虽然我给你的previos步骤将保存文件在您的下载文件夹，但他们不会显示在这个菜单上，因为他们没有下载。然而，使这一工作，您必须启动文件下载，以便在此处显示文件。
以下是如何开始下载的示例：

DownloadManager.Request request = new DownloadManager.Request(uri);
request.setDestinationInExternalPublicDir(Environment.DIRECTORY_DOWNLOADS, "fileName");
request.setNotificationVisibility(DownloadManager.Request.VISIBILITY_VISIBLE_NOTIFY_COMPLETED); // to notify when download is complete
request.allowScanningByMediaScanner();// if you want to be available from media players
DownloadManager manager = (DownloadManager) getSystemService(DOWNLOAD_SERVICE);
manager.enqueue(request);

此方法用于从URI下载，我还没有将其用于本地文件。
如果您的文件不是来自互联网，您可以尝试保存一个临时副本，并为此值获取文件的URI。

只需使用DownloadManager下载生成的文件，如下所示：

File dir = new File("//sdcard//Download//");

File file = new File(dir, fileName);

DownloadManager downloadManager = (DownloadManager) context.getSystemService(DOWNLOAD_SERVICE);

downloadManager.addCompletedDownload(file.getName(), file.getName(), true, "text/plain",file.getAbsolutePath(),file.length(),true);

"text/plain"是你传递的mime类型，所以它会知道哪些应用程序可以运行下载的文件。

我使用以下代码在我的Xamarin Droid项目中使用C#实现了这一点：

// Suppose this is your local file
var file = new byte[] { 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20 };
var fileName = "myFile.pdf";

// Determine where to save your file
var downloadDirectory = Path.Combine(Android.OS.Environment.ExternalStorageDirectory.AbsolutePath, Android.OS.Environment.DirectoryDownloads);
var filePath = Path.Combine(downloadDirectory, fileName);

// Create and save your file to the Android device
var streamWriter = File.Create(filePath);
streamWriter.Close();
File.WriteAllBytes(filePath, file);

// Notify the user about the completed "download"
var downloadManager = DownloadManager.FromContext(Android.App.Application.Context);
downloadManager.AddCompletedDownload(fileName, "myDescription", true, "application/pdf", filePath, File.ReadAllBytes(filePath).Length, true);

现在，您的本地文件被“下载”到您的Android设备上，用户会收到一个通知，并且一个对该文件的引用被添加到下载文件夹中。不过，请确保在写入文件系统之前向用户请求权限，否则将抛出“访问被拒绝”异常。

适用于API 29及以上

根据documentation，要求使用Storage Access Framework作为Other types of shareable content, including downloaded files。
应使用系统文件选取器将文件保存到外部存储目录。
将文件复制到外部存储示例：

// use system file picker Intent to select destination directory
private fun selectExternalStorageFolder(fileName: String) {
    val intent = Intent(Intent.ACTION_CREATE_DOCUMENT).apply {
        addCategory(Intent.CATEGORY_OPENABLE)
        type = "*/*"
        putExtra(Intent.EXTRA_TITLE, name)
    }
    startActivityForResult(intent, FILE_PICKER_REQUEST)
}

// receive Uri for selected directory
override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {
    super.onActivityResult(requestCode, resultCode, data)
    when (requestCode) {
        FILE_PICKER_REQUEST -> data?.data?.let { destinationUri ->
            copyFileToExternalStorage(destinationUri)
        }
    }
}

// use ContentResolver to write file by Uri
private fun copyFileToExternalStorage(destination: Uri) {
    val yourFile: File = ...

    try {
        val outputStream = contentResolver.openOutputStream(destination) ?: return
        outputStream.write(yourFile.readBytes())
        outputStream.close()
    } catch (e: IOException) {
        e.printStackTrace()
    }
}

由于Oluwatumbi已正确回答了此问题，但首先需要检查是否通过以下代码为WRITE_EXTERNAL_STORAGE授予了权限：

int MY_PERMISSIONS_REQUEST_WRITE_EXTERNAL_STORAGE = 1;

if (ContextCompat.checkSelfPermission(context, Manifest.permission.WRITE_EXTERNAL_STORAGE)
            != PackageManager.PERMISSION_GRANTED) {
        // Permission is not granted
        // Request for permission
        ActivityCompat.requestPermissions(thisActivity,
                new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE},
                MY_PERMISSIONS_REQUEST_WRITE_EXTERNAL_STORAGE);

    }else{
        Uri uri = Uri.parse(yourUrl);
        DownloadManager.Request request = new DownloadManager.Request(uri);
        request.setDestinationInExternalPublicDir(Environment.DIRECTORY_DOWNLOADS, "fileName");
        request.setNotificationVisibility(DownloadManager.Request.VISIBILITY_VISIBLE_NOTIFY_COMPLETED); // to notify when download is complete
        request.allowScanningByMediaScanner();// if you want to be available from media players
        DownloadManager manager = (DownloadManager) getSystemService(DOWNLOAD_SERVICE);
        manager.enqueue(request);
    }

这是使用Xamarin android API版本29的DownloadManager运行代码下载文件

public bool DownloadFileByDownloadManager(string imageURL)
        {
            try
            {
                string file_ext = Path.GetExtension(imageURL);
                string file_name = "MyschoolAttachment_" + DateTime.Now.ToString("yyyy MM dd hh mm ss").Replace(" ", "") + file_ext;
                var path = global::Android.OS.Environment.DirectoryDownloads;

                Android.Net.Uri uri = Android.Net.Uri.Parse(imageURL);
                DownloadManager.Request request = new DownloadManager.Request(uri);
                request.SetDestinationInExternalPublicDir(path, file_name);
                request.SetTitle(file_name);
                request.SetNotificationVisibility(DownloadVisibility.VisibleNotifyCompleted); // to notify when download is complete

                // Notify the user about the completed "download"
                var downloadManager = DownloadManager.FromContext(Android.App.Application.Context);

                var d_id = downloadManager.Enqueue(request);
                return true;
            }
            catch (Exception ex)
            {
                UserDialogs.Instance.Alert("Error in download attachment.", "Error", "OK");
                System.Diagnostics.Debug.WriteLine("Error !" + ex.ToString());
                return false;
            }
        }

在API 29和30上工作

要获取目录，请执行以下操作：

Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS);

在您的AndroidManifest.xml中设置此权限：

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

将requestLegacyExternalStorage添加到AndroidManifest.xml中的应用程序标记（适用于API 29）：

<application
    ...
    android:requestLegacyExternalStorage="true">