php8新特性，如何理解使用单问号空运算符？

$a = ($name=="苏南大叔")?"ok":"sorry";

sn = null ?? "苏南大叔";  // 苏南大叔

x = y ? z运算;

Parse error: syntax error, unexpected token ";" 

class Blog{
    public function __construct(public string $domain){

    }
    public function getUrl(){
        return "https://".$this->domain."/";
    }
}
class SuNan{
    public function __construct(public string $name,private string $domain){
        $this->Blog = new Blog($domain);
    }
    public function getBlog(){
        return $this->name == "苏南大叔" ? $this->Blog : null ;
    }
}

$ss = new SuNan(domain:"newsn.net",name:"苏南大叔");
$nn = $ss->getBlog()?->getUrl();

$ss2 = new SuNan("sunan大叔","newsn.com.cn");
$nn2 = $ss2->getBlog()?->getUrl();

var_dump($nn);
var_dump($nn2);

string(18) "https://newsn.net/"
NULL