"It's like a film set. Like it's not real," the bemused Uber driver says as we pass through a succession of silent rural villages washed with milky sunlight. He's right: signs of human life are weirdly absent during our windy journey through the Alentejo. I'm making my way to Ai Weiwei's secluded farm, nestled deep in the Portuguese countryside, where arguably the world's best known living artist has quietly lived for the last six years.

When I arrive, I am ushered inside a vast cathedral of a building, a replica of Ai's erstwhile, and controversially demolished, studio in Shanghai. The space is cavernous. It's gridded by huge wooden columns and its dizzyingly high ceilings are webbed with beams. There is no artwork on the walls, nor are there any obvious signs of production or activity.

"People say, what is this building for? what? Fuck that. Maybe it's nothing," Ai tells me with a shrug. "It's empty. And emptiness, people cannot accept."

Ai has been busy filling other vacant spaces: I'm visiting him to chat about the unveiling of his latest work, Mushrooms, this year's Neil Balnaves Tallawoladah Lawn Commission outside Sydney's Museum of Contemporary Art. Characteristically epic, elliptical and urgent, this installation is 40 metres in length, and consists of 500 identical cast iron sculptures of combat helmets on an elevated platform. The title alludes to the visual rhyme between mushroom caps and the shape of the helmets – as well as the fungal associations with decay, death and the fragility of life, brutally exposed by armed conflict.

"Curiosity" and "boredom" generally shape Ai's creativity, and the origins of this particular commission are "very simple". In 2015, Ai began work on Human Flowhis documentary on the migrant crisis for which he and his team travelled to 23 countries. The process of producing this film led him to reflect more deeply on the causes of these perilous mass movements across the globe: war – or more specifically, the mechanics and munitions involved in contemporary warfare.

"I studied for two years to understand the military, arsenals, who designs bombs, and how they are used. Then, when Russia threatened Ukraine and Ukraine asked for help from Germany, the German chancellor offered them 5,000 helmets. I laughed," Ai says. "I thought, wow, that's interesting. So, at the beginning of the conflict, you can say Germany or Europe did not understand the nature of the war."

Last year, Ai also visited Ukrainian soldiers on the frontline four times. He scrolls through his iPhone and shows me pictures and footage he took during his time there: Ai being taught how to use a rifle, the troops sporting uniforms he had embroidered with motifs of sunflowers. He plans to use these clips in an upcoming film.

But he is keen that I do not simply see these experiences and this background as research for the new MCA piece. "I never prepare my life so I can make an artwork – or create artwork for my life," he says. Mushrooms was also shaped by its future home, "a beautiful location" that he has visited several times, with a dramatic view of the Sydney Opera House. That interested him too: "That building represents rational, peaceful enjoyment … but at the same time we have to re-announce that there are wars going on and peace is essential for any civilised society."

His choice of words – "re-announce" – is striking. Throughout our conversation, over green tea, Ai seems despairing about a collective and cold amnesia about the parlous state of our geopolitics; if not amnesia, our deliberate refusal to truly engage with the doomed state of our world. "We cannot be blind, we cannot say I don't know the situation," he says. "You know the situation and you just pretend you don't know it." When speaking about the genocide in Gaza, he seems, too, deeply concerned about the lack of connectedness within modernity, our failure to understand that "if injustice happens to one person, it happens to the whole of humanity as well". Much of his art, not least his trip to the battlefields of Kharkiv, seems an attempt to reestablish a connection and commitment to other human beings, however difficult and dangerous that might be.

Can art play some role in offering him hope about the future of humanity? No, he says, typically pugnacious and plain speaking: "Art today is very corrupted – 90% of what we see in most institutions, or in so-called artistic circles, I think is just garbage. Real art questions the existing order. Art is not about representing so-called beauty, but rather to question the existing judgment about everything in politics, in life, in personal relations, in individual judgment – but this kind of art is never encouraged. People want you to behave, want you to be a nice guy, pleasant, cheerful, enjoying your life. And so most work we see in the museums is very decorative and sentimental, rather than having any relations to human struggle and our reality."

The Chinese word for mushroom is 蘑菇 or mógū, a term often associated with slowness, delay and dawdling. While our discussion regularly returned to Ai's dissatisfaction with the tardy pace of societal and institutional progress, the conversation itself was leisurely. Despite describing himself as "the most interviewed human being" and claiming to have been interviewed "more than 1,000 times", he is generous, fleetingly mischievous – and often startlingly wise. I'm curious, after Mushrooms, where his interests might take him next. But his existence is not so teleological: he has no "goals".

"The only thing we should try to do," he says, "is live a life which can naturally capture the light."

• Ai Weiwei's Mushrooms is free to visit now on the Tallawoladah Lawn at the Museum of Contemporary Art in Sydney