public function get($id)

MagentoSalesModelOrderRepository

public function get($id)
{
    if (!$id) {
        throw new InputException(__('Id required'));
    }
    if (!isset($this->registry[$id])) {
        /** @var OrderInterface $entity */
        $entity = $this->metadata->getNewInstance()->load($id);
        if (!$entity->getEntityId()) {
            throw new NoSuchEntityException(__('Requested entity doesn't exist'));
        }
        $this->setShippingAssignments($entity);
        $this->registry[$id] = $entity;
    }
    return $this->registry[$id];
}

$this->metadata->getNewInstance()->load($id);

MagentoSalesModelResourceModelMetadata

MagentoSalesModelOrder

$this->loadWithCustomAttributes();