That's a huge question - but in my view it really boils down to four things:
1) A clear sense of the problem they were trying to solve;
2) Crystal clear focus on only the most high-impact things;
3) Relentless pursuit of excellence in execution.
4) Luck.

That fourth one mightn't be a popular one to state, but the truth is that there are a lot of companies who try to do 1 through 3, but 4 always comes into play and Google is no different.