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.