Migrating native BillingClient and StoreKit code to shared Kotlin Multiplatform in-app purchases

The text discusses the process of migrating from native Android and iOS in-app purchase implementations to a shared layer using Kotlin Multiplatform and RevenueCat's SDK. It highlights the challenges of integrating billing systems because of the differing purchase models and types on each platform. The migration process involves replacing native billing initializers, purchase flows, and entitlement checks with Kotlin Multiplatform equivalents, simplifying the codebase by using RevenueCat's store-agnostic concepts like Offerings, Packages, Entitlements, and CustomerInfo. This transition allows for a unified purchase flow and easier management of entitlements across both platforms, while maintaining continuity for existing customers through synchronization of purchases and user identities. The text also outlines the importance of verifying parity between the native and shared implementations before removing the old code and notes some platform-specific capabilities that require additional consideration.