Neos.ContentRepository/Classes/Domain/Model/Node.php
public function getPrimaryChildNode(): ?NodeInterface
    {
        return $this->nodeDataRepository->findFirstByParentAndNodeTypeInContext($this->getPath(), null, $this->context);
    }
public function getPrimaryChildNode(): ?NodeInterface
    {
        return $this->nodeDataRepository->findFirstByParentAndNodeTypeInContext($this->getPath(), null, $this->context);
    }
public function getPrimaryChildNode ( ) : ? NodeInterface { return $ this -> nodeDataRepository -> findFirstByParentAndNodeTypeInContext ( $ this -> getPath ( ) , null , $ this -> context ) ; }
Returns the primary child node of this node.

Which node acts as a primary child node will in the future depend on the
node type. For now it is just the first child node.

@return NodeInterface|null The primary child node or NULL if no such node exists
@deprecated with version 4.3, without any replacement.
Returns the primary child node of this node .