I also mentioned "communicates using oral and written language". Could have also listed a shared propensity to make and use tools.. I mean you can pick whatever set of traits you like that is sufficiently descriptive to represent "humanoid". The final destination is the same. And the point was that you are choosing to follow this reductionist path with halflings as if it is only applicable for them, and I feel that is a waste of energy.
Tool use and communicating with language (both oral and written) also applies to Mindflayers and Ogres. So, not sure why you felt the need to expand your list.
But again, you are using the broadest possible terms, things that are obviously going to be true of basically everything, and then trying to twist that around into stating that there is no continuum. That there is no "more or less" but that since at the farthest end of the spectrum everything is the same that there is no point in looking towards the middle of the spectrum and noting a lack of difference or more differences.
Yes. If you want to keep the list to "thinks, uses tools, speaks, has a head, arms and legs" then halflings, elves, dragonborn and humans are all identical. You can even add "mortal, corporeal, bipedal, has forward facing eyes" and they are still identical. But that doesn't mean there is basically no difference between an Illithid and a halfling.
As far as I know there have been no direct observations of Dragonborn facial tissue, bone structure, vocal apparati, etc. (Not even considering how magic might come into play). Artist's depictions of fantasy creatures are, unfortunately, not proof of their biology. I don't really disagree with you that your conclusion may be a reasonable one. But, while it could be a reasonable conclusion, it is not the only possible reasonable conclusion, and it certainly isn't a "factual" conclusion.
So, the only possible way to interact with the fiction is not proof of anything within the fiction. If this were true, DnD itself would not exist. You would literally be unable to have a shared fictional world for people to interact with if it was impossible to determine things about a shared fictional world through words and pictures.
"Artists depictions" is how we catalogued things before Cameras. And, shockingly, photography is still considered an art. So, in one sense, every single thing you have seen a picture of to determine if it is based in reality is "an artists depiction". I suppose you could take the stance that anything you cannot directly observe with your own eyes is potentially false, but then you have hallucinations, Descartes, so at some point you need to go forward with the assumption that things are true.
Fantasy is literally "made up". Worse than that as it relates to D&D, it's made up and has magic. It is 100% subjective and unmoored from the constraints of reality. There is no meaningful consensus. Hell, you posted a bunch of art for halflings and gnome with wildly different characteristics as imagined by different artists. No one is going to be proven wrong for imagining something differently than another person does. Do you really believe otherwise??
You miss the point by trying to make my own point more extreme than it is. You'd be able to tell if I posted a picture of a lizardfolk instead of a gnome right? If I came to you and said "this is a gnome, just like in DnD" and it was a picture of an alligator, you'd tell me I was wrong, right?
If there is no meaningful consensus, how could you tell me what dragon is? IF you say that the party is fighting a dragon and the barbarian says they step on it and squish it, because there is no consensus and they think of small depictions like Psuedodragons or St. George's Dragon, then are they right? Or would you say that they are clearly acting out and not following how things are meant to be? Because, shockingly, we do actually have a meaningful consensus of what being a dragon means in DnD. To the point that we have other "dragon related monsters" that have different names. Because they aren't dragons.
So, yes, I do believe that a meaningful consensus can be built about a fantasy world. The point I was making about halfling and gnome art was to showcase that it really isn't so cut and dry. Many of the pictures people posted as "halflings" I thought were gnomes. So relying on "physical descriptions" to differentiate halflings from the rest of the world... has an issue.
As it relates to building materials, would expect a combination of both differing materials and different usage. Stone, wood, and clay are the materials harvested for building because they are the materials that work to address normal building engineering requirements. You change those requirements, you change the population of harvestable materials. Balsa might be an extreme example, but sure, that might be one. The other piece would be things like board thicknesses as you surmised. Wooden trusses used to support a bridge are larger/thicker/heavier than the ones used to support your roof. It's the same principle, if you don't have to do as much with it, you don't have to use as much of it.
Okay, yeah, you are using the word "materials" in a very different manner than I am. You are thinking more about the shapes, thickness, and other elements while I was literally thinking about the actual material itself. Stone is stone, if you are building a building out of stone that can withstand the weather and keep out a wild beast, you are going to be using the same types of stone, even if you can make them thinner or shape them differently due to weight variances.