The array structure is a little complex:
$res = array( 'headers' => array(), 'response' => array( 'code' => int, 'message' => string ) );
All of the headers in $res['headers'] are with the name as the key and the value as the value. So to get the User-Agent, you would do the following.
$user_agent = $res['headers']['user-agent'];
The body is the raw response content and can be retrieved from $res['body'].
This function is called first to make the request and there are other API functions to abstract out the above convoluted setup.
Request method defaults for helper functions: - Default 'GET' for wp_remote_get() - Default 'POST' for wp_remote_post() - Default 'HEAD' for wp_remote_head()