How can I import a type declaration from another folder?

Question

I want to separate my type declarations into separate folders like:

/types/index.d.ts
/types/express/index.d.ts

However, I lose the typings if I move the type definitions for express out of the root /types/index.d.ts file and import from a folder.

/types/index.ts

import './express'

/types/express/index.d.ts

// what do I export?
declare global { // is this correct?
  namespace Express {
    export interface Request {
      user: User & {
        id: string;
      };
    }

    export interface Response {
      clearAuthCookie: () => this;
    }
  }
}}

My ts.config:

...
"typeRoots": ["node_modules/@types", "types"]