async methods using or not using the threadpool, being scalable or not

Question

can you please help me understand the difference between the two methods here:

1) why does the first use the thread pool and the second doesn't?

2) Why does the second give you scalability, but the first doesn't - is this related to thread pools?

3) How can I call the two methods in main to make their purpose more obvious (and better high light their differences)?

4) I know async Task methods can be called with await, what about Tasks that lack async like these methods?

public class _12_MyClass
{

    public Task SleepAsyncA(int millisecondsTimeout)
    {
        return Task.Run(() => Thread.Sleep(millisecondsTimeout));
    }

    public Task SleepAsyncB(int millisecondsTimeout)
    {
        TaskCompletionSource<bool> tcs = null;
        var t = new Timer(delegate { tcs.TrySetResult(true); }, null, -1, -1);

        tcs = new TaskCompletionSource<bool>(t);
        t.Change(millisecondsTimeout, -1);

        return tcs.Task;
    }

    public static void Main()
    {
    }
}