Why is cooking a food item method called different things by what the item is, or what is the criteria?

Boiling and poaching are not the same. Frying and sautéing are not the same.

Yes, there are differences between those things.

Poaching is cooking in hot liquid, but the liquid is not boiling or even simmering, so it is a lower temp than both.

Saute generally means you're using a small amount of oil/fat and stirring/tossing the food to spread the oil/fat around on everything while cooking everything. Pan frying generally means you're cooking a larger piece of something and not tossing it around.

As pointed out already, these terms have quite specific and different meanings but it's worth being aware that they often get used interchangeably a lot too, and therefore possibly incorrectly. A lot of the specific terms will come from french cuisine (like saute) including all different names for exactly how you've sliced up your vegetables (julliene, brunoise etc). I believe this was so recipes could be written very precisely and therefore reproduced more acccurately.

It usually has to do with what chemical process happens to the food in question. Not all foods react the same to being dunked in boiling water. (Although I couldn't tell you what the difference between potatoes in boiling water and carrots in boiling water.) In the case of onions vs eggs, the same process is 1) extracting the water and using it to make sauce, with the onions, or 2) boiling off a tiny amount of liquid and heating the proteins to solidify them, in the case of eggs. Same method, wildly different chemistry.

Sometimes it has to do with how long that cooking method is applied, since a different thing happens. For example, you can poach OR hard-boil an egg; same method, different amount of cooking time.

In short, with a few exceptions, it's not about what process you're applying to cook the food, but about the result that it achieves in the food item.

Poach equals lower temp liquid. It can be oil or water type liquid. Boil is maximum temp water type liquid only. Blanching is boiling for a short time with the intention of not cooking all of the way through (eg to get skin off or to prepare for preservation by freezing).

Fry and saute are used interchangably all the time. One person's fried onions is another's sauted onions. Saute should indicate small pieces turned or tossed in a moderate amout of fat. Fry can be small or large pieces and can have moderate to lots of fat as a cooking medium.

Note that this is how normal people in my region and life use these terms and I make no claim that this is 'right' just my experience.

It's not based on the item. You can boil turkey and poach eggs as well. Here's various definitions to get you started:

https://www.webstaurantstore.com/article/454/types-of-cooking-methods.html

I have no answers, just pointing out that boiled carrots and poached eggs are also things.

Boiling, poaching and blanching all have to do with how long something is immersed in boiling water, for instance. (I think, and those definitions may also subtly change with the food item.)

Boiling happens in boiling liquid, poaching happens at a lower temperature and you wouldnt boil turkey. Sauteing is frying but uses little fat, unlike deep frying for example.

They're slight variations in technique. Poaching is usually at a lower temperature than boiling, for instance.

There has to be a guide about this stuff somewhere on the web, right?

Well, until this post, I had never heard of poached carrots

i think you lack cooking technique. all of these things are different.. ie;
Poaching refers to cooking in hot—not boiling—water