You've designed high-stakes systems.
Uber: Geospatial indexing for nearby drivers. State machine for ride lifecycle. Real-time location updates.
Payments: Idempotency prevents double-charging. Two-phase auth/capture. Double-entry ledger. Daily reconciliation.
E-commerce: Inventory reservation with timeouts. Saga pattern for distributed transactions.
These systems prioritize correctness over performance. Next, you'll design data-intensive systems like Google Docs.