Lots of different problems that are complex! For example:

+ Text internationalization system

+ Payment processing

+ User auth

+ User verification

+ A/B testing system

+ User metrics collection from UI

+ offline storage of said metrics

+ data pipelines to process stored offline storage

+ iOS app

+ android app

+ Image storage and caching infra

+ fraud detection

+ search ranking and indexing

https://medium.com/airbnb-engineering/airbnb-engineering-inf...