Contractor SWE's at Google are very rare. In my several years there as both a SWE and EM (I'm no longer at Google), I never encountered one. My understanding from other EM's/Directors was they were only hired for non core product work, which did not overlap with the work that Google FTE SWE's did, and was very routine work. Most likely without internal code access - it was that separate.

Core product/infra work, and anything non-trivial, is always done by FTE's.