$lookup is this a true left outer join?

Hi, can someone help me understand why aggregation on orders collection referenced in the link below with $lookup pipeline having from as inventory collection returns documents with null sku or no sku element from inventory collection? Isn’t that $lookup acts as a left outer join where in orders collection is the left outer collection here? I was expecting left outer join to perform something like this: https://tutorialdeep.com/sql/sql-left-outer-join/

         from: "inventory",
         localField: "item",
         foreignField: "sku",
         as: "inventory_docs"

$lookup example here - https://docs.mongodb.com/manual/reference/operator/aggregation/lookup/

Oops, looks like $lookup is working as I expect it to, but I have missed to see an empty document in the orders which has caused this confusion. I have re-visited the documentation and everything looks fine.