Google does have another outlet, which is acquisitions. When a company is acquired, generally, a big chunk of the employees get hired without the normal interview process. Some of them are essentially on probation, and if they perform well, they are made permanent. Others end up not getting converted.
Google also hires contractors and interns, and some of those get 'converted' to full time employees, essentially with the contracting/intern period as one big job interview.
Whether the employees get interviewed at Google or not depends on the size of the acquisition, at least for engineering positions. That's largely for logistical reasons. The vast majority of acquisitions of software engineers involve interviewing.
Google also hires contractors and interns, and some of those get 'converted' to full time employees, essentially with the contracting/intern period as one big job interview.