How a program is executed asynchronously:

Question

I found an Asynchronous code as shown below. In this code we are trying to read the content of each URL mentioned in the List asynchronously. As per my understanding, when the Async method is called and the method is Awaited control is returned back to the calling thread.

So my doubt is "What actually happens behind the scene when reading contents of the URLs are run asynchronously.. Are the individual tasks running on separate threads or are they executed parallelly on different CPU cores or something else?"

 class Program
{
    static readonly HttpClient s_client = new HttpClient
    {
        MaxResponseContentBufferSize = 1_000_000
    };

    static readonly IEnumerable<string> s_urlList = new string[]
    {
        "https://learn.microsoft.com",
        "https://learn.microsoft.com/aspnet/core",
        "https://learn.microsoft.com/azure",
        "https://learn.microsoft.com/azure/devops",
        "https://learn.microsoft.com/dotnet"
    };

    static Task Main() => SumPageSizesAsync();

    static async Task SumPageSizesAsync()
    {
        var stopwatch = Stopwatch.StartNew();

        IEnumerable<Task<int>> downloadTasksQuery =
            from url in s_urlList
            select ProcessUrlAsync(url, s_client);

        List<Task<int>> downloadTasks = downloadTasksQuery.ToList();

        int total = 0;
        while (downloadTasks.Any())
        {
            Task<int> finishedTask = await Task.WhenAny(downloadTasks);
            downloadTasks.Remove(finishedTask);
            total += await finishedTask;
        }

        stopwatch.Stop();

        Console.WriteLine($"\nTotal bytes returned:  {total:#,#}");
        Console.WriteLine($"Elapsed time:          {stopwatch.Elapsed}\n");
    }

    static async Task<int> ProcessUrlAsync(string url, HttpClient client)
    {
        byte[] content = await client.GetByteArrayAsync(url);
        Console.WriteLine($"{url,-60} {content.Length,10:#,#}");

        return content.Length;
    }
}