Is comment fields data store in render array? Is render array contain key-value entries that is generated by node module only? or comments fields also included?
The render array for a node contains also the content of the comments, which is added by comment_node_view(), an implementation of hook_node_view(). That hook is invoked from node_build_content(), which is called from node_view().

The Devel module shows the following output for a node render array.



In Drupal 8, node_view() is a thin wrapper for entity_view(), which renders the node using entity_render_controller().