db={"coll1":[{"_id":ObjectId("63074885ff3acbe0d63d7686"),"year":"2020","energy_products":"Other Energy Products","sub_products":"Other Energy Products","value_ktoe":"70.4"}],"coll2":[{"_id":ObjectId("63074882ff3acbe0d63c391a"),"year":"2020","energy_products":"Petroleum Products","sub_products":"Other Petroleum Products","value_ktoe":"10633.7"}]}

db.coll1.aggregate([{"$group":{"_id":{"energy_products":"$energy_products"},"avg":{"$avg":{"$toDouble":"$value_ktoe"}}}},{"$unionWith":{"coll":"coll2","pipeline":[{"$group":{"_id":{"sub_products":"$sub_products"},"avg":{"$avg":{"$toDouble":"$value_ktoe"}}}}]}}])