Other guy did a good job on the main points, but I'll add something I saw in a study on a kind of bird in the US:

The birds realised cigarette butts had an antibacterial effect, and made efforts to collect and use cigarette butts in their nest building for eggs and chicks.

Learning and making use of novel materials.

Other animals that build stuff use natural materials. Humans are the only ones that process raw materials into different materials and build with those.

Wrong on both counts. First, animals that build stuff don’t just use “natural materials” they use whatever is available to them. Birds make nests out of everything from sticks to metal hangers and from moss to our “unnatural” polyester products they can get their hands on.

Second, bees and ants and termites and wasps etc. use raw materials like fibers (“natural” or not) or pollen or grains of clay and sand and typically mix them with their saliva or water or other bodily excretion or all of those together to create novel building materials.

Animals don’t create stuff with iron (scorpions and certain sea snails actually do kinda use metallic iron) or plastic (technically many “natural” materials are polymers aka plastics) not because we are the only ones capable of understanding resource machines, but because we are in the sweet spot where many tools are available to us. We are large enough to work with fire and hammers. If termites could make steel they absolutely would, but they can’t. They make concrete though because they can. Diatoms make glass (which most other living things can’t do) because they can. Ants farm and domesticate “resource machines” like fungi or plants or other insects (kinda like we do) because they can. We just happened to be in the sweet spot for making our own resource machines without needing to wait for evolution to evolve them.

You could argue that a wooden hut with a thatched roof is a natural structure, but not much else in human architecture

I think most houses even just a few hundred years ago would be “natural” even by your definition.

Clay and mortar are just rocks we mix with water rather than saliva like insects would. Wood from trees like beavers. Slate shingles from, well, slate. The only real issue would be glass for windows and that is a naturally produced resource, we just produce it in an easier way than diatoms do (we actually kinda use their skeletons funnily enough along with geologically occurring silicate sands ofc) and voila you have a relatively modern house. Nails would require a long process but good news you don’t need them to build a house, they just make things easier.

Most of our old civilizations last so long as ruins because they’re made out of stone, sure we mined that stone but so do ants and termites. The roads the Romans built are just as natural as ant mounds are and so are the pyramids (minus the gold caps at the top perhaps).

Humans are different but in a way similar to flying animals or similar, our intelligence gives us access to cumulative technology more than any one person can do in their lifetime, it makes us a different category but still part of the system.

If we are nature, anything we make is natural.

Unless you are saying that for it to be natural it has to he made using a biological process, perhaps inside the body?