At the beginning of 2017, ~12,000 of Google's current ~72,000 employees had previously worked at other big tech companies, and most of them were probably in their R+D headcount of only 27,000. I wouldn't be at all surprised if half of their engineering interviewees came from similar paying jobs, and a the majority of the remainder were college hires.

https://www.alphr.com/business/1005261/silicon-valley-swipin...

https://www.statista.com/statistics/273744/number-of-full-ti...

https://www.statista.com/statistics/219333/number-of-google-...