Chain method php

Sekarang saya lagi membuat project "QueryBuilder", fitur yang sama kek di laravel.

Ini file utamanya:

class Query extends DB {

    private static $tb;
    private $query_stmt;
    private $key;
    private $trigger_code;

    public static function table(string $arg) {
        self::$tb = $arg;
        return new static;
    }

    public function select(array $column) {
        $column_stmt = "";
        if(is_array($column)) {
            foreach($column as $q) {
                if($q != $column[array_key_last($column)]) {
                    $column_stmt .= $q.",";
                } else {
                    $column_stmt .= $q;
                }
            }
            $this->query_stmt = "SELECT $column_stmt FROM ".self::$tb;
            return $this;
        }
    }
     
    public function where(array $arg) {
        if(is_array($arg)) {
            $this->key = $arg[array_key_first($arg)];
            $this->query_stmt .= " WHERE ".array_key_first($arg). " = ?";
            $this->trigger_code = 1;
            return $this->exec();
        }
    }

    public function like(array $arg) {
        if(is_array($arg)) {
            $this->key = $arg[array_key_first($arg)];
            $this->query_stmt .= " WHERE ".array_key_first($arg). " LIKE ?";
            $this->trigger_code = 2;
            return $this->exec();
        }
    }

    public function exec() {
        $stmt = $this->koneksi->prepare($this->query_stmt);
        if($this->trigger_code == 1) {
            $stmt->execute([$this->key]);
            return $stmt->fetch(PDO::FETCH_ASSOC);
        } else if($this->trigger_code == 2) {
            $stmt->execute(["%".$this->key."%"]);
            return $stmt->fetchall(PDO::FETCH_ASSOC);
        }
    }

}

Ini cara penggunaan, menggunakan select sekaligus where 

$x = Query::table("dataapi")
            ->select(["id","username"])
            ->where(["username" => "ahmad"]);
print_r($x); 

// Hasil nya berhasil 
// Outputnya seperti ini
Array ( [id] => 29 [username] => Ahmad )

tapi, ketika saya menggunakan fitur select doang

$x = Query::table("dataapi")
            ->select(["id","username"]);

print_r($x);

Outputnya kaya gini:

Query Object ( [koneksi:protected] => PDO Object ( ) [query_stmt:Query:private] => SELECT id,username FROM dataapi [key:Query:private] => [trigger_code:Query:private] => )

Kenapa seperti itu ya?
ekspektasi saya sih, harusnya me-return hasil keseluruhan.

Maaf kepanjangan pertanyaannya, soalnya saya bingung mau nanya dimana lagi

Di method select dia masih return ke object itu sendiri, artinya yaa belum ada action apapun yg dijalanin, sedangkan di method where itu akan di lanjutkan ke method exec.
Kalo tujuan akhirnya mau muncul resultnya, tinggal gini aja :

$x = Query::table("dataapi")
            ->select(["id","username"])->exec();

print_r($x);