IMHO the "eras" of Google were:
1.) 1998-2001. Origins. Larry was CEO, company was still basically in the garage or the PayPal offices in Palo Alto, everything was an ad-hoc scrappy startup.
2.) 2001-2004. Growth. Eric was CEO. They moved into the Googleplex. Exploring new lines of business (Groups, AdSense, GMail, Maps/Earth), but they were all just explorations, and most of the company remained focused on Search. Key infrastructure like GFS, MapReduce, BigTable, Stubby, Sawzall were invented.
3.) 2004-2008. Conglomeration. Eric was CEO. IPO. Big college-hiring push started in this era (remember the Google Labs Aptitude Test?), as did a push to hire outside managers from other companies. GMail, Maps, Labs, Scholar, Toolbar, Chrome, Android all launched, and Docs/Sheets/Slides/YouTube were acquired. This is when the modern Google product portfolio was built, and much of its current culture and processes solidified.
4.) 2009-2010. Interregnum. Eric was CEO. Financial crisis happened here, which basically shut down growth, and it also faced serious competitive threats from Bing, iPhones, etc. As a result the culture during the interregnum almost harkened back to the 2001-2004 era: there was a refocusing on core search/ads/infrastructure and a generally scrappier mentality.
5.) 2011-2015. Google+ & X. Larry was CEO again, company shifted to make everything about G+. There was an overall futile attempt to address a major competitive threat that turned out not to matter anyway.
6.) 2015-2020. Alphabetization. Sundar was CEO, but Ruth came in as CFO and realistically runs the company. Finance plays a much bigger role - I don't think Google's produced a single successful new product post-2015 (come to think of it, there weren't many new successes post-2009), but revenue has grown through continued optimization and profits have grown through cost-cutting.
7.) 2020+. Post-COVID. Culture was hurt significantly by remote work, it's very hard to realign the company to adapt to outside changes.
The irony is that what many Googlers nostalgically think of "Google culture" actually came from the 2004-2008 era, when Eric was CEO and many of the new hires either came from college or outside companies. If you listen to pre-IPO hires, they'll say that very early Google culture was a lot scrappier, a lot less entitled, a lot more opinionated, a lot more willing to take top-down direction from Larry specifically, and a lot more focused on the work itself and how to deliver excellent technical solutions quickly.