Step 3 calls self.with_context() with some keyword arguments. This returns a new version of self (which is a product.product recordset) with the keys added to the
current context. We are adding two keys:

location: This one is mentioned in the docstring of the product.product method computing the qty_available field.
active_test: When this key is present and linked to the False value, the search() method does not automatically add ('active', '=', True) to the search domain. Using this ensures that in step 4, we get all the products, including the disabled ones.

When we read the value of product.qty_available in step 5, the computation of that field is made using only the specified stock location.