Contact -> User, User -> Contact hook_views_data_alter()

gistfile1.txt

$data['civicrm_contact']['user'] = [
          'title' => 'User related to the CiviCRM contact',
          'help' => 'Relate user to the CiviCRM contact.',
          'relationship' => [
            'base' => 'users_field_data',
            'base field' => 'uid',
            'first field' => 'contact_id',
            'second field' => 'uf_id',
            'id' => 'civicrm_entity_civicrm_contact_user',
            'label' => 'User',
          ],
        ];

        $data['users_field_data']['civicrm_contact'] = [
          'title' => 'CiviCRM contact related to the user',
          'help' => 'Relate CiviCRM contact to the user.',
          'relationship' => [
            'base' => 'civicrm_contact',
            'base field' => 'id',
            'first field' => 'uf_id',
            'second field' => 'contact_id',
            'id' => 'civicrm_entity_civicrm_contact_user',
            'label' => 'CiviCRM contact',
          ],
        ];