Nginx はどのようにして http から https に自動的にジャンプしますか?

server {
    listen 80;
    listen [::]:80;
    return 301 https://$host$request_uri;
}

server {
    listen 443 ssl;
    listen [::]:443 ssl;

    ssl_certificate         certificate_file_path;
    ssl_certificate_key  certificate_key_file_path;

    ...

}

server {

    if ($server_port = 80 )   

    #if ($scheme = http )

    #if ($ssl_protocol = "")

    {
        return 301 https://$host$request_uri;
    }

    
    listen 443 ssl;
    listen [::]:443 ssl;

    ssl_certificate         certificate_file_path;
    ssl_certificate_key  certificate_key_file_path;

    ...

}