Most stopwatch.dll implementations rely on the Win32 API function QueryPerformanceCounter (QPC) to achieve microsecond or nanosecond precision.
A standard tool for .NET files that shows the underlying code structure and metadata. stopwatch.dll
Tools like .NET Reflector or ILSpy can reconstruct the original source code from the DLL. Common Use Cases Stopwatch Class (System.Diagnostics) - Microsoft Learn Most stopwatch