function custom_blocks_theme($existing, $type, $theme, $path) {
  return [
    'custom_blocks__front_apps' => [
      'variables' => [
        'app' => NULL
      ],
    ],
  ];
}

/**
   * {@inheritdoc}
   */
  public function build() {
    return [
      '#theme' => 'custom_blocks__front_apps',
      '#app' => 'test value',
    ];
  }

<p>Hello: {{ app }}</p>

/**
    * Implements hook_theme() to add the template definition.
    **/
    function lotus_theme($existing, $type, $theme, $path) {
      return [
        'lotus_template' => [
          'variables' => ['test_var' => NULL],
        ],
      ];
    }

public function build() {

    return array[
      '#theme' => 'lotus_template',
      '#test_var' => $this->t('Test Value'),
    ];

  }

<p> This is the lotus template with a value of {{ test_var }} </p>