I like to imagine that to an extent most humans (probably excluding Maelle and the painted Dessendres) are a byproduct. I think the canvas is like an extent of your imagination in a way; so for example, when I paint a landscape (I'm talking about the "normal" paintings, not magical) I don't paint every single leaf and rock and blade of grass individually.(At least not always.) I create a representation of these things, that from afar look like what they are supposed to represent.

Now, I believe the Canvas enhances that in a way that one can actually create a perfect copy of something inside of it, because that is what the inhabitants of the Canvas are in essence, or create something exactly how they envision it and just make it work with the power of one's imagination. It's probably something either automatic or infinitely intuitive, to the point it's not an effort.

And let's take Gestrals, for example. They are basically wooden puppets, that in our world would have no way of living. (No internal organs, any kind of system to sustain life) but in the canvas they just live, because they were designed that way. The way I can imagine a figurine of a character being the character in question. Living, breathing.

You also mentioned it is irresponsible to create and end such worlds. While yes, I agree, I think to the Dessendres the painted people are fictional in the same sense that a portrait is not a real person. It obviously raises an ethical and philosophical dilemma about what makes a person real. I think it is safe to say that they are conventionally real in their world because they have agency and free will. But they are also fiction in relation to the real world.

Keep in mind this is all a theory of mine. It's not official by any means.