Cant download large files in browser

I'm using a POST request with body to dynamically create a specified file on the server side and streaming it to the client as a response. The problem on the client side is that the file is being downloaded into the response object (browser's cache memory I suppose, as a temporary file) instead of the PC's hard drive.

How can I pass the byte stream directly for the browser to handle and direct into the /Downloads folder or wherever the user has set the downloads go?

The file size is really large and there's not enough space on the gadget's hard drive to create it there and give a link for GET request. So I need to create it on the fly and stream to the client as a POST response.

Right now I've written my code in Angular, but the language doesn't really matter. I'd like to understand how the browser handles the situation.
When I execute the HTTP request, I bet the browser does something with it before it's passed to the socket. And I'm sure the browser does something with the received request too before passing it to the piece of code which performed the request.

I mean can you only by setting the response headers make the browser download the file into /Downloads? What happens then to the waiting HTTP request if the packets are "stolen" by the browser? Or when the response headers arrive to the response object, can you somehow notify the browser that the body of the response is a file byte stream we wish to download into /Downloads?

And I have the possibility to modify my server code too.

EDIT More detailed explanation:
My server is sending the 5GB file to the client. Instead of just receiving that file in the temporary memory of the browser I would like to save the file directly on hard drive.
But naturally the browser prevents the direct access to the users computer so I can't write it there. So how do I pass the byte stream for the browser to handle and save into a file?