Before Google there used to be shitty search engines like Altavista and Yahoo!, and there were many of them so you had to also use a "meta" search engine which was basically a program running locally on your computer which would take your search query and forward it to a dozen search engines and then shows you the aggregated results. That was one way of combining their strengths let's say since each one of them was complete shit.

The results were still shit though because many websites were gaming those search engines as SEO at the time was extremely easy: the search engines simply looked at your meta tags (where you could spam your keywords) and the keyword density of your pages.

Then Google came with its PageRank algorithm and obsoleted the meta tags altogether. Keyword density became also less important. Google basically assigned a score manually to a dozen trustworthy and high quality websites and then let those scores propagate with some decay through its graph representing all webpages it indexed and the links between them, so if a website A with a PageRank of 10 for example linked to your website B, you'd inherit part of that PageRank (how much will depend on how many outgoing links website A has, the more outgoing links it has, the less your website B will get). It was basically a measure of trustworthiness/quality and they then ranked the webpages in their results mainly according to that score.

Things went amazingly well for a few years and no one missed the old search engines, then the SEO community found a way to abuse that new algorithm again and the idea was very simple: massively exchange links and even buy them from platforms like TextLinkAds (it's dead now but you could look it up on Wikipedia). So we went back to the shitty results again.

Then you also have another big trouble maker: Google AdSense. The idea of this thing was to pay website owners if they accept to display Google's ads and they'd get paid something proportional to the number of clicks/impressions the ads would get on their website. The concept was okay, website owners could make some money, Google also wins, and the ads were mostly textual and none of the annoying popup ads you'd see at the time. Then it didn't take long for people to abuse that system too, especially with people like Joel Comm shilling the idea of making websites purely for AdSense and retiring from them, people began creating spammy websites with garbage content that's filled with keywords just so that they can put Google AdSense ads on them, those websites were called "Made For Adsense" (MFA), and that immediately polluted the search results because you started having millions of them.

Sure Google made improvements later on and incorporated AI to have the search engine also understand the content of the webpages, which in theory should help with relevance, but due to the cat & mouse between Google and the SEO (& the MFA) community things are still shitty and the only way you can get very good results today is if you insert a site:stackoverflow.com or site:reddit.com at the end of your search query.