I haven’t tested your code but I would expect it to work better like this:

public ActionResult LoadVideo()
{
    return Content("http://another-server.com/123.mp4"); //send URL of Video to caller
}

Then also your HTML would be like this :
(where src calls the LoadVideo function and uses the returned response).

<source src="@Url.Content("LoadVideo")" type="video/mp4" />

How is it generate an actionresult in ASP.NET MVC.

public ActionResult LoadVideo()
{
    string file="http://another-server.com/123.mp4";// file is URL Video 
    // how to response a video partial for view in <Video></Video> HTML
}

I want view result in HTML

<video width="400" controls>
  <source src="/Controller/LoadVideo" type="video/mp4">
 
  Your browser does not support HTML5 video.
</video>

1. Video is big, how can it send as partial to client browser?
2. Video is 720 , how can convert to 480 and send to browser client?
3. Is it possible to convert .m8u3 ?