On a misty Saturday afternoon in Shenzhen’s Central Park, a gaggle of teenage girls are sheltering from the drizzle under a concrete canopy. With their bags of crisps piled high in front of them, they crowd around a couple of smartphones to sing along to Mandopop ballads. The sound of their laughter rings out across the surrounding lawn – until it is pierced by a mechanical buzzing sound. Someone has ordered dinner. A few metres away from the impromptu karaoke session is an “airdrop cabinet”, one of more than 40 in Shenzhen that is operated by Meituan, China’s biggest food…