Using the new features SafeFileHandle for IO operations

Question

Can anybody help me with the new SafeFileHandle feature in .NET 6?

I tried this article, but the output isn't merging; only a single file.

Here's my old code using File.Create() only:

public static async Task MergeFiles(string folderToMerge, string outputFilePath) 
{
    using var output = File.Create(outputFilePath);
    var files = folderToMerge.EnumerateFiles("mp4");
    foreach (var (i, file) in files.EnumerateWithIndex()) 
    {
        Console.WriteLine($"Merging file {i + 1} of {files.Count()}");
        using var input = File.OpenRead(file);
        await input.CopyToAsync(output);
    }
}