Routing in Drupal 8 for basic pages
Node routes are declared using what is known as a route provider.
The node route provider class is \Drupal\node\Entity\NodeRouteProvider and it is declared in the annotation of the node class:

/*
 * @ContentEntityType(
 *   id = "node",
 *   label = @Translation("Content"),
 *   bundle_label = @Translation("Content type"),
 *   handlers = {
 *     ...
 *     "route_provider" = {
 *       "html" = "Drupal\node\Entity\NodeRouteProvider",
 *     },
 *     ...
 */
class Node extends ContentEntityBase implements NodeInterface {
//  ...
}