Create new node and assign it to user after they have been registered I have one user role on d8 and have few additional fields on user account so when people are registering they can leave additional data. What I would like to achieve is that when user registers - new content of that specific type would be automatically created (node), assigned to him and the fields on that new content would be populated from those fields on user account which he filled during the registration process.
I was thinking that this is maybe achievable with the Rules but since couldn't find proper documentation for Rules in d8 not sure is this is doable that way.
This is very easy to do in D8 Rules. Here is an export of a Rule which does what you asked - when a new user is created, it creates a new node (of type article, as an example). It then sets the node author to be the new user. It then sets a field of the new node (the body field, in this case) to a value take from the user (the user's email address, in this case). As you can see, Rules are stored in YAML files, and are imported/exported like any other configuration in D8. So you can just import this and you will immediately have a working example which you can then modify to use the specific fields you've defined on your user entity and node type. This Rule additionally displays a system message, which is a very useful way to debug Rules.

langcode: en
status: true
dependencies: {  }
id: create_a_node_for_each_new_user
label: 'Create a node for each new user'
events:
  -
    event_name: 'rules_entity_insert:user'
description: 'Node fields will be populated from values entered on the user form.'
tags: {  }
config_version: '3'
expression:
  id: rules_rule
  uuid: c9428f2d-c0b9-47bf-a7e5-3f2cc8af9f54
  conditions:
    id: rules_and
    uuid: 47b8d622-ed38-4977-8e58-4cca6d89260a
    conditions: {  }
  actions:
    id: rules_action_set
    uuid: d96fd6b5-1638-4b15-adde-5f5d7673fb8f
    actions:
      -
        id: rules_action
        uuid: e09cd52d-76fe-4682-91de-bfed58a651ab
        context_values:
          message: 'Node created using information entered by &lt;em&gt;{{ user.name }}&lt;/em&gt; when registering'
          type: status
          repeat: false
        context_mapping: {  }
        context_processors:
          message:
            rules_tokens: {  }
          type:
            rules_tokens: {  }
          repeat:
            rules_tokens: {  }
        provides_mapping: {  }
        action_id: rules_system_message
      -
        id: rules_action
        uuid: fe2acb55-9234-4754-9d2b-3d1889d1775c
        context_values:
          type: article
          title: 'Article created by Rules for {{ user.name }}'
        context_mapping: {  }
        context_processors:
          type:
            rules_tokens: {  }
          title:
            rules_tokens: {  }
        provides_mapping: {  }
        action_id: 'rules_entity_create:node'
      -
        id: rules_action
        uuid: 743be03f-d1b7-4251-a1ac-80b17aa5c86e
        context_values:
          value: '{{ user.mail.value }}'
        context_mapping:
          data: entity.body
        context_processors:
          value:
            rules_tokens: {  }
        provides_mapping: {  }
        action_id: rules_data_set
      -
        id: rules_action
        uuid: 91f3c8f2-eb47-46e6-a9dd-2affc90e15d2
        context_values: {  }
        context_mapping:
          data: entity.uid.target_id
          value: user.uid.value
        context_processors: {  }
        provides_mapping: {  }
        action_id: rules_data_set