Create new node and assign it to user after they have been registered
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