If you take a look at the e2e/tsconfig.e2e.json file, you'll see that it starts with the following line: "extends": "../tsconfig.json".
The file might be called tsconfig.json if you're using a more recent version of the Angular CLI!
Using this directive, you can inherit from another TypeScript configuration (and even chain multiple ones). This is very useful for larger projects where only specific settings differ in some parts of the system.
You can read more about this here: https://www.typescriptlang.org/docs/handbook/tsconfig-json.html#configuration-inheritance-with-extends.