Low-level design defines classes, interfaces, invariants, state transitions, and collaborations inside a component. Start with use cases and domain rules; do not turn every noun into a class automatically. Give each module a cohesive responsibility and let the owner of mutable state enforce its invariants.

Encapsulation prevents callers from creating invalid states. An order should expose meaningful operations such as confirm() or cancel() instead of unrestricted setters that permit impossible combinations. Composition is often preferable to deep inheritance because behavior can vary independently and tests can replace collaborators.

Use class diagrams for static relationships and sequence diagrams for runtime interactions. Show where domain decisions, persistence, network calls, and side effects occur. A strong design makes business rules testable without requiring every test to start a database or call a remote service.

SOLID principles are heuristics for reducing change coupling. Single Responsibility seeks one cohesive reason to change; Open/Closed encourages extending stable behavior without repeated edits; Liskov Substitution preserves subtype contracts; Interface Segregation keeps interfaces focused; Dependency Inversion separates policy from volatile implementation details. Applying an interface to every class can add needless indirection, so use abstractions where change, testing, or multiple implementations justify them.

Common patterns include Strategy for interchangeable algorithms, Factory for varying construction, Adapter for incompatible interfaces, Observer for event notification, Decorator for layered behavior, and Command for representing an operation as data. Patterns are useful when they clarify variation; they are not goals in themselves.

State machines are particularly useful for payments, orders, uploads, and durable workflows. Define valid states, transitions, triggers, side effects, and terminal states. Persist important transitions and make side effects idempotent because an in-memory state machine alone cannot recover after a process restart.