One of the big differences is that at some point they seemed to basically delist forums, de-rank most blogs, and totally forgot that personal web pages still exist. All those things are still out there, but you are highly unlikely to find them via Google now. Maybe if you do an exact phrase search for the page's title, otherwise, they're buried.

Another is that they tried to get too helpful and it backfired. There used to be a bunch of search operators that you could use to be specific and most of them don't work anymore. Because that was too complicated for most people. So now instead Google just ignores what you searched for and tries to guess what you actually meant, and ends up showing a lot of irrelevant results, some of which don't match what you searched for at all.

The third is that nowadays almost always, Google tries to spin it into a commercial request. The top results are usually for e-commerce sites or something selling products or services, no matter what you search for. It always assumes you're trying to buy something, not trying to find information.

I switched to duckduckgo years ago, and it generally gives much better results, but even it seems to be slipping a bit now. Still way better than the modernized Google, but for how long?

The results are worse because the Internet is worse.

Exactly, the incentive to produce good quality content is no longer there.