Is it correct that a node and it's translations have the same node id? I just translated a node and I saw that both nodes have the same node id. Since I cant find anything about it: is this correct?
Yes it's correct because Drupal 8 use Entity Translation (in core) for the translations.

You can more about this in Drupal 8 multilingual tidbits 17: content translation basics


  If you already built multilingual sites with Drupal 7 that had content
  translation, you may notice this model is a refined version of the
  Drupal 7 Entity translation module (without awkward modules required
  like Title) rather than the Drupal 7 core content translation module.
  The core module in Drupal 7 keeps different copies of nodes and
  relates them together in a translation set. In Drupal 8 this feature
  is not available, instead you can configure translatability on a field
  (and in some cases subfield) basis. However if you configure all
  fields to be fully translatable, you can essentially reproduce the
  Drupal 7 behavior. Compared to the Drupal 7 core solution with
  different entity identifiers then, Drupal 8 will have the same entity
  identifier but a different language variant.
  
  The biggest advantage of the Drupal 8 solution is you can configure to
  translate as much as needed and not more. With one unified solution to
  translate only some fields or all fields, (contributed) modules only
  need to deal with one system instead of two. With storing translations
  under one entity, modules that don't want or need to deal with
  multilingual scenarios can still consider the entity as one, no need
  for special translation set support. That is way more flexible than
  Drupal 7 and best of all it applies to all kinds of content, not just
  nodes. It is future proof for modules you enable like rules and
  commerce.