This video was really confusing because you don't explain what a key is.
What is this "Key"? It looks like TKey is the same as when we used a dictionary. But why would Keys have anything to do with this because we aren't using a dictionary we are using lists and objects? Lists don't even have named items we could call a key.
Steven Parker173,991 Points
The "key" here isn't part of a data structure, but the field to be used in the "Join" operation to match with a similar field in the other data source.
Can you explicate a little? What field? Fields are part of objects. I don't know if the items of a list are fields though. And as far as I can tell we are matching on values rather than variables. And what does this lambda have to do with a key/field?