How do I combine filtering inside aggregate with project in mongoose?

I am trying to find the document that fits a specific field then take all the results and then project to all of them a rating based on an equation.
I tried first finding the fitting documents with a field then sorting it, however I do not not know how to combine them and use the results from the first query to the second one:

router.get('/questionsfilter',
  body("whichTag").trim().escape(),

  function(req,res,next){
    Question.find({tag: req.body.whichTag}).exec(function(err,results2){
      try{
        let now = new Date();
        const results = await Question.aggregate([
          {$project: {rating: {$add:[
            {$divide: 
              [
                {$divide: [{$arrayElemAt:[{$ifNull:["$views",[0]]}, 0]},  {$subtract:[now, '$dateCreated']}]},
                50
              ]},
            {$subtract: 
              [
                {$multiply: [{$divide: [{$arrayElemAt:[{$ifNull:["$likes",[0]]}, 0]}, {$subtract:[now, '$dateCreated']}]}, 3]},
                {$multiply: [{$divide: [{$arrayElemAt:[{$ifNull:["$dislikes",[0]]}, 0]},  {$subtract:[now, '$dateCreated']}]}, 6]}
              ]
            }
            ]}
          }},
          {$sort: {rating:-1}},
          {$limit: 15}
        ]).exec();
        const questions = await Promise.all(results.map(({_id}) => Question.findOne({_id}).exec()));
        res.render('content', { whichOne: 5, user: req.user, questions:questions, type:'hot', whichTag: req.body.whichTag});
      }
      catch(err){
        next(err);
      }
    })
  }
);

how would I find all the questions’s whose tag field fits the req.body.whichTag then sorting them with first projecting a rating then with map? I cant find out how to combine both and only how to do one!
Thanks!