Securing broader healthcare coverage at lower costs remains a pressing challenge worldwide. In response, FinTech has facilitated a novel sharing economy model, online mutual aid (OMA), which provides large scale quasi-health insurance against critical illnesses in China with over 100 million users and at a low annual cost of less than 20 dollars per person. We investigate the economics of OMA using all claim cases and approximately 200,000 members randomly drawn from a representative OMA platform. Despite concerns regarding OMA's financial sustainability due to its ex-post cost-sharing structure, there is compelling evidence of progressive cross-subsidies and advantageous selection. We also find that the inclusive but relaxed enrollment standards of OMA and the resulting adverse selection are counterbalanced by internal measures including varying indemnities to different age cohorts and rigorous claims investigations. Notably, during the COVID-19 pandemic, OMA claim processing time increased by only 4.25%, while traditional insurance companies nearly stopped processing altogether, highlighting the resilience and efficacy of the technology. We estimate that OMA has the potential to save up to $306.87 billion annually, extending $53.75 trillion worth of healthcare benefits to the entire population under age 70 in China. OMA therefore holds potential to significantly enhance the accessibility and affordability of healthcare, especially during public health crises.