The payment rails problem nobody mentions in UBI debates

Everyone debates whether UBI is affordable. Almost nobody asks how you'd actually PAY billions of people every month. As a fintech PM this is the part that keeps me up: identity (who gets it, exactly once, without excluding people who lack documents), rails (bank transfers? wallets? cash agents?), and fraud at a scale no bank has ever handled. Brazil's Pix and India's UPI are the closest proof that instant, near-free rails work at hundreds of millions of users. Any credible UBI needs that layer built BEFORE the funding question matters. What should the payout stack look like?