What if you have to link to a specific url in in twig, but you only have the nid? Moreover, it is good practice to use the id and not a hardcored url. Here's what you should use:
Sometimes it is needed for a user to be redirected after registration. The user is not logged in yet, and has to go to a landing page with more instructions on verifying his e-mail.
foreach (\Drupal::routeMatch()->getParameters() as $entity) {
if ($entity instanceof \Drupal\Core\Entity\EntityInterface) {
// here $entity is your object
}
}