What are the Apex Enterprise Patterns (Selector, Domain, Service, Application)?

Apex Enterprise Patterns (from the fflib open-source framework) provide a structured, scalable architecture:

1. Selector Layer: Centralises all SOQL queries for an object. Each selector class queries the object and enforces consistent field sets and FLS. Prevents repeated, inconsistent queries across the codebase.
2. Domain Layer: Encapsulates business logic for a specific object (the "domain"). Trigger handlers call the domain class, which contains methods like onBeforeInsert().
3. Service Layer: Coordinates cross-object business logic and orchestrates calls between domain and selector layers. Called from triggers, LWC, REST endpoints.
4. Application Factory: A factory class (using the Selector/Domain/Service interfaces) that allows dependency injection and mocking in tests. This pattern makes large codebases maintainable, testable, and consistent.