when should I explicitly import type in TypeScript?

Question

I'm taking the type-challenge from https://github.com/type-challenges/type-challenges/blob/master/questions/898-easy-includes/README.md,and I find something interesting and confusing: Why files in the same directory could use type without import and export ? My project is like :

and I define sometype in template.ts

// template.ts
type Includes<T extends unknown[], K> = T extends [infer First, ...infer Rest]
  ? K extends First
    ? true
    : Includes<Rest, K>
  : false

and in the some directory ,

// test-cases.ts
import { Equal, Expect } from '@type-challenges/utils'

type cases = [
  Expect<
    Equal<Includes<['Kars', 'Esidisi', 'Wamuu', 'Santana'], 'Kars'>, true>
  >,
  Expect<
    Equal<Includes<['Kars', 'Esidisi', 'Wamuu', 'Santana'], 'Dio'>, false>
  >,
  Expect<Equal<Includes<[1, 2, 3, 5, 6, 7], 7>, true>>,
  Expect<Equal<Includes<[1, 2, 3, 5, 6, 7], 4>, false>>,
  Expect<Equal<Includes<[1, 2, 3], 2>, true>>,
  Expect<Equal<Includes<[1, 2, 3], 1>, true>>,
  Expect<Equal<Includes<[{}], { a: 'A' }>, false>>,
  Expect<Equal<Includes<[boolean, 2, 3, 5, 6, 7], false>, false>>,
  Expect<Equal<Includes<[true, 2, 3, 5, 6, 7], boolean>, false>>,
  Expect<Equal<Includes<[false, 2, 3, 5, 6, 7], false>, true>>,
  Expect<Equal<Includes<[{ a: 'A' }], { readonly a: 'A' }>, false>>,
  Expect<Equal<Includes<[{ readonly a: 'A' }], { a: 'A' }>, false>>,
  Expect<Equal<Includes<[1], 1 | 2>, false>>,
  Expect<Equal<Includes<[1 | 2], 1>, false>>,
  Expect<Equal<Includes<[null], undefined>, false>>,
  Expect<Equal<Includes<[undefined], null>, false>>
]

you can see that I didn't export type Includes and neither import it , In test-cases.ts file I could use it . but if change the template.ts with importing some util type from other directory:

// template.ts
import { Equal } from '@type-challenges/utils'

type Includes<T extends unknown[], K> = T extends [infer First, ...infer Rest]
  ? Equal<First, K> extends true
    ? true
    : Includes<Rest, K>
  : false

I got error in test-cases.ts,it says can't find the name Includes

could someone explain why is that?