Putting objects inside a Canvas changes their position - Unity 2D

So i've got a canvas set to Screen Space and Scale with Screen Size in my Unity 2D project. Whenever I make something a child of the canvas, it changes their position by a lot. Like if something is in position 11, 4 it makes it like 261, 108. I'm assuming theres like some conversion it makes, but why is that? I never learned about canvases and I'm very new to this, so an explanation would help!

Also, if I change a gameObjects position in code, say I set its X position to 3, but that gameObject is in the Canvas, how come it changes it's position to the actual location I want, and not to the "3" in the canvases space?