If you're qualified to work for a team that's hiring, transfers are pretty easy. I work for GOOG outside CA and know a lot of coworkers who transferred teams within the company from out of state. Lateral mobility is something of a perk.
You have to make sure to join in the bay area though. Offers in my local office are typically for 1/11 the initial equity of offers for the same level in Mountain View, San Francisco, or Sunnyvale, and I was not permitted to speak to any hiring managers in the US after seeing a four-day exploding offer from my local office.
I'm surprised you received an exploding offer; I had thought that was something we didn't do?
Pay depends enormously on the local market for your skills. NYC and Boston pay almost as much as Mountain View, while (ex) London pays much less. People moving between offices are often offered large increases/decreases. A strategy of joining in Mountain View and then transferring to a lower-paying office isn't generally going to give you much higher pay at the new office (though you'll of course have the money you put away while in Mountain View).
Working in Mountain View for 6 months is worth literally a million dollars of (MV initial equity - local office initial equity), assuming I was going to work in the local office for the remaining 3.5 years either way.
> Does anyone know what proportion of their workforce is not in California?
I think it's about half.
> is it possible to move to say Boulder within Google easily?
Reasonably. You need to find a team at the other office that has openings for someone with your skills, but I know many people who have done it (Mountain View -> Cambridge, NYC -> Seattle, Cambridge -> Pittsburgh, Mountain View -> Boulder among others).
https://about.google/locations/?region=asia-pacific&office=s...
Does anyone know what proportion of their workforce is not in California?
A question to Googlers and those who have worked there, is it possible to move to say Boulder within Google easily?