I'm an ex-Googler, and my guess is that > 50% of engineers work on something directly related to advertising. There are just so many projects and systems and moving parts needed to make all that money, it might be easy to underestimate how much engineering effort it takes. And Google management knows very well what it is that makes the company so profitable.

Non-advertising projects get lots of attention, but below the surface is a hulking mass of engineers supporting new ad formats, new ad optimizations, accounting systems, analysis tools, ad targeters, advertiser support front-ends, etc.

Corollary: If you go to work for Google, the odds are that you will be writing code to support advertising.

(I could be wrong--maybe it's less than 50%. But it's a lot, and my guess is that your "vast majority" is a much less educated guess than mine is.)