[{"main":"xxx","phone_numbers":["+1234567890","+0987654321"],"branch":[{"company":{"name":"c_name1","head":"M.J","location":{"address":"address1","phone":"phone1"},"url":"name1.com"},"fo_date":"2019","title":{"name":"c_name1","role":"marketing","respons":["marketing","business administration"]},"location_names":["loc_name1","loc_name2"],"summary":"summary1"},{"company":{"name":"c_name2","head":"M.J","location":{"address":"address2","phone":"phone1"},"url":"name2.com"},"title":{"name":"c_name1","role":"marketing","respons":["marketing","business administration"]},"location_names":["loc_name3","loc_name4"],"summary":"summary1"}],}]

db.collection.aggregate([{$project:{main:"$main",phone_numbers:{$reduce:{input:"$phone_numbers",initialValue:"",in:{$concat:["$$value","$$this","; "]}}},}}])