Outsourcing? Not even a little bit. Not only that but AFAIK contract engineers are incredibly rare and nonexistent in many product areas.
This isn't to say that Google doesn't have partnerships with external vendors. We do. But engineering in general is a core competency, something far too valuable and of strategic importance to outsource.
There are exceptions. But for software engineers who contribute to Google's (non-open source) code base, Google pretty much got out of using contractors around 2008 or so. Some contractors became employees.
This isn't to say that Google doesn't have partnerships with external vendors. We do. But engineering in general is a core competency, something far too valuable and of strategic importance to outsource.