Stock price appreciation. Say that your pay is $150K base and a further $150K in RSUs, vesting over 4 years and with periodic refresh grants. Google stock has tripled over the last 5 years, so by the time 5 years has rolled around, assuming you keep your stock, your effective compensation was $600K/year ($150K/year salary and stock grants currently worth $450K/year) for that first year and slightly less for subsequent years (because the refresh grants are offered at a higher price). After taxes & living expenses you've got maybe $1.5-2M in the bank.

No, this is not guaranteed going forwards, and yes, an outsider could achieve the same effect just putting their paycheck into Google stock. Someone who works at the company has inside information about whether that's a good idea, though; if you're an outsider, it really is a crapshoot whether the stock goes up or down, while if you're an insider, you have knowledge of what upcoming projects are happening, how much key metrics are growing, what morale is like, etc.

This is also why I say it could turn around very fast - if your employer is Google and much of your net worth is Google stock, then if Google falls on hard times you could be wiped out just as easily as your nest egg grew.