This repository is a curated collection of low-level design (LLD) and machine coding round solutions, primarily targeting interview preparation. It contains over 15 self-contained subprojects, each modeling a classic system design problem such as chess, elevator control, Uber ride booking, Splitwise expense splitting, parking lot, stock broker, task scheduler, Kafka message queue, in-memory key-value store, and more. Each subproject follows a consistent architecture with domain models, repository interfaces with in-memory implementations, service classes, and often a Main or Demo class for standalone execution. The code heavily employs design patterns like Strategy, Factory, Repository, and Observer, and demonstrates concurrency handling with locks, atomic variables, and thread-safe collections. The collection is not intended for production deployment; all state is volatile and stored in-memory. While the quality is uneven across subprojects, many exhibit clean separation of concerns and extensible design. The repository serves as a practical study resource for object-oriented design, with problem statements and video tutorials linked in the README. Some subprojects have multiple iterations (e.g., Kafka from simple to production-grade) showing evolution, but this also introduces duplication and occasional inconsistencies in naming and package structure. Despite the lack of automated tests and incomplete implementations in a few areas, the overall codebase demonstrates solid craftsmanship for educational purposes. A handful of modules—such as the distributed cache with consistent hashing, the circuit breaker for resilience, the order matching engine, and the heap-based feed fetch strategy—are particularly well-isolated and could be extracted for reuse in other projects with minimal adaptation.