I want to join/group differents collections in one

[MaBeuLux88_xxx (Maxime Beugnet)]

@MaBeuLux88_xxx

  {
    '$unwind': {
      'path': '$subscribers'
    }
  }, {
    '$group': {
      '_id': null, 
      'subs': {
        '$push': '$ROOT'
      }
    }
  }, {
    '$project': {
      '_id': 0, 
      'subs': 1
    }
  }

Try my version with this at the end.