Drupal 8 - Programatically create new fields

basefields.php

<?php

/**
 * Implements hook_entity_base_field_info().
 */
function mymodule_entity_base_field_info(EntityTypeInterface $entity_type) {
  if ($entity_type->id() == 'user') {
    $fields = [];

    $default_field_definition = [
      'type'        => 'string',
      'label'       => t('My application field'),
      'description' => NULL,
      'displays'    => [
        'form' => FALSE,
        'view' => TRUE,
      ],
      'settings'    => [
        'max_length' => 255,
      ],
      'read-only'   => TRUE,
    ];

    $custom_fields_definitions = [
      'account_id'        => [
        'label'       => t('My application account ID'),
        'description' => t('The unique ID on My application.'),
        'settings'    => ['max_length' => 60],
        'constraints' => ['UniqueField'],
      ],
      'account_name'      => [
        'label'       => t('My application account name'),
        'description' => t('The account name on My application.'),
        'settings'    => [
          'max_length'     => 60,
          'case_sensitive' => TRUE,
        ],
        'constraints' => ['UniqueField'],
      ],
      'username'          => [
        'label'       => t('My application username'),
        'description' => t('The account name on My application.'),
        'settings'    => [
          'max_length'     => 60,
          'case_sensitive' => TRUE,
        ],
        'constraints' => ['UniqueField'],
      ],
      'email'             => [
        'type'        => 'email',
        'label'       => t('My application account email'),
        'description' => t('The email on My application.'),
      ],
      'first_name'        => [
        'label'       => t('First name'),
        'description' => t('The first name from My application.'),
      ],
      'last_name'         => [
        'label'       => t('Last name'),
        'description' => t('The last name from My application.'),
      ],
      'score' => [
        'type'        => 'integer',
        'label'       => t('My application score'),
        'description' => t('Number of votes for this user on My application.'),
        'settings'    => ['unsigned' => TRUE],
      ],
      'is_pro'       => [
        'type'  => 'boolean',
        'label' => t('Pro enabled'),
      ],
    ];

    $myapp_fields = [];
    foreach ($custom_fields_definitions as $name => $field) {
      $myapp_fields['myapp_' . $name] = $field + $default_field_definition;
    }

    foreach ($myapp_fields as $field_name => $field_info) {
      $field_definition = BaseFieldDefinition::create($field_info['type'])
        ->setLabel($field_info['label'])
        ->setDescription($field_info['description'])
        ->setReadOnly($field_info['read-only']);

      foreach (($field_info['displays'] ?? []) as $key => $value) {
        $field_definition->setDisplayConfigurable($key, $value);
      }

      foreach (($field_info['settings'] ?? []) as $key => $value) {
        $field_definition->setSetting($key, $value);
      }

      foreach (($field_info['constraints'] ?? []) as $constraint) {
        $field_definition->addConstraint($constraint);
      }

      $fields[$field_name] = $field_definition;
    }

    return $fields;
  }
}