TypeScript's built-in utility types and how to build custom ones — Partial/Required, Pick/Omit, Readonly/Record, Extract/Exclude, NonNullable/Awaited, ReturnType/Parameters/ConstructorParameters/InstanceType, key remapping with `as` in mapped types, and hand-rolled custom mapped types. Use when picking the right built-in utility type instead of hand-writing one, extracting a function's parameter or return type, filtering a union with Extract/Exclude, or remapping object keys.