How to fix Module 'X' cannot be ambient in this context in TypeScript

// ❌ WRONG - in a regular .ts file
declare module 'some-external-module' {
  export interface SomeInterface {
    name: string;
  }

  export function doSomething(): void;
}

export function myFunction() {
  // implementation
}

// ✅ CORRECT - in a .d.ts file
declare module 'some-external-module' {
  export interface SomeInterface {
    name: string;
  }

  export function doSomething(): void;
}

src/
├── types/
│   └── module-declarations.d.ts      # Module type definitions only
├── lib/
│   └── my-module.ts                  # Implementation code
└── index.ts

declare module 'custom-module' {
  export interface Config {
    apiUrl: string;
    timeout: number;
  }

  export function initialize(config: Config): void;
}

import type { Config } from 'custom-module';

export function setupApp(config: Config) {
  // implementation using the declared types
  console.log('Connecting to ' + config.apiUrl);
}

// ✅ CORRECT - in a .d.ts file
declare module 'express' {
  interface Request {
    userId?: string;
    userRole?: string;
  }
}

// ✅ CORRECT - augmenting with new exports
declare module 'react' {
  export interface Component {
    customMethod?(): void;
  }
}

// src/lib/my-utils.ts - Implementation
export function formatDate(date: Date): string {
  return date.toISOString();
}

export function parseDate(str: string): Date {
  return new Date(str);
}

// src/lib/my-utils.d.ts - Type declarations
export function formatDate(date: Date): string;
export function parseDate(str: string): Date;

{
  "compilerOptions": {
    "declaration": true,
    "declarationDir": "./dist/types",
    "outDir": "./dist",
    "rootDir": "./src",
    "typeRoots": ["./node_modules/@types", "./src/types"],
    "skipLibCheck": true
  },
  "include": ["src/**/*"],
  "exclude": ["node_modules", "dist"]
}