How is data sent from link and script?

I

Ingerniated2018-02-24 01:59:20

Web development

Ingerniated, 2018-02-24 01:59:20

Hi everyone, please advise.
When we receive a request to the server, we open the file and get this text, but

<!doctype html>
<html>
<head>
<link rel="stylesheet" type="text/css" href="css.css">
</head>
<body>
...
</body>
</html>

How is data sent from css?
The server opens the html, then opens the css and inserts into the place of the link those lines of code that are in it, or does it simultaneously send all the files at once and only when all the files are received will the browser show us the page?