db={"coll1":[{"_id":"5ecf3445365eca3e58ff57c0","type":"city","name":"Toronto","CSD":"3520005","CSDTYPE":"C","PR":"35","PRNAME":"Ontario","geometry":"Object","country":"CAN","updatedAt":"2021-04-23T18:25:50.774+00:00","province":"ON"}],"coll2":[{"_id":"5ecbe871d8ab4ab6845c5142","geometry":"Object","name":"henry12","user":"5cbdd019b9d9170007d15990"}]}

db.coll1.aggregate([{"$unionWith":{"coll":"coll2"}},{"$project":{"_id":"$_id","name":"$name","type":{"$cond":[{"$ne":[{"$type":"$type"},"missing"]},"$type","custom"]},"province":{"$cond":[{"$ne":[{"$type":"$province"},"missing"]},"$province",null]}}},{"$sort":{"name":1}}])