What are utility types like Partial, Pick, and Omit?

Question presented to candidate: "TypeScript ships utility types like Partial, Pick and Omit. What do they do, how are they built, and when would you reach for each one in real code?"

What a strong answer should cover:

• They are generic types in the standard library that derive a new object type from an existing one, so one source type stays the single source of truth
• Partial<T> makes every property optional, Required<T> makes every property mandatory, Readonly<T> makes every property readonly, Pick<T, K> keeps keys K, Omit<T, K> drops keys K
• They are mapped types over keyof T (Omit is Pick plus Exclude), and they are shallow: nested objects are not changed
• Typical uses: update patches (Partial<Omit<User, "id">>), create inputs (Omit<User, "id">), previews (Pick), and combining them (make only some keys optional)
• Traps: Omit does not check its keys, Omit on a union collapses it to the shared keys, and Partial lets a spread overwrite a value with undefined

Clarifying questions expected:

• Should the derived type change if the source type changes (derive it), or is it a separate contract (write it out)?
• Is exactOptionalPropertyTypes on? It changes what an optional property may hold

Code / implementation expected: Optional — derive a patch and a create type from one interface, or write Partial by hand as a mapped type.