A few people own a lot of automated machinery that hypothetically makes everything including procuring raw materials (farming, mining etc.). They make enormous amounts of money and the taxes they pay essentially pays for the entire populace. Alternatively the government owns the means of production. There’s a smaller secondary economy where new ideas are tested and eventually filter up to automated mass production if it’s good enough. It’s obviously a pipe dream but it could all happen very quickly if there is some kind of true AGI event.