$data = [
            'id' => 1,
            'relations' => [
                [
                    'id' => 100,
                ],
            ],
        ];

        $ci = $this->serializer->denormalize($data, Ci::class);

use Doctrine\Common\Collections\ArrayCollection;
use Doctrine\Common\Collections\Collection;

/**
 * @template-extends Collection<int, Relation>
 */
class RelationCollection extends ArrayCollection
{

}

final readonly class Ci
{
    public function __construct(
        public int $id,
        /**
         * @var RelationCollection<Relation>
         */
        public ?RelationCollection $relations = null,
    ) {}
}

final readonly class Relation {
    public function __construct(
        public int $id,
    )
    {
    }
}