Why are Finder search results so terrible?

You aren't doing anything wrong. That's the way it works.

The problem is the platform you are using. You are posting in the Ventura forum. That means you are using a Mac. That means your hard drive is full of things called "folders" that you've created so that you can organize your files.

But the modern Mac is simply an iPhone with a built-in keyboard and large display. A few years ago, Apple even tried to change the default Finder display to "all my files". Do you remember that? The default view was literally a list of 300,000 files, all folders and organization removed. They backed off on that, but only a little.

This is the way.

All that being said, I can't reproduce your results. When I do a new search, it does default to the entire machine. But if I narrow it down to a specific folder, then it searches only that folder ands it children.

Also, you've said that you "keep updated as always", yet your signature says you are running macOS 11.2. That's not updated, that's far behind. I'm not saying that will fix the search. Far from it. It will make your Mac even more like an iPhone. But search is not something that has changed much, for better or worse, in many years.

Also, it kind of sounds like you are doing a Spotlight search, not a Finder search. Finder only searches files. Spotlight will search files, emails, texts, and other stuff.

It sounds like search is functioning properly; the files returned are not irrelevant as they include whatever it is you searched for. You can, of course, add additional limiters, like "kind" or a date. You can also organize your brach results by folder, type, etc.