type checking custom AppConfig entries

https://nuxt.com/docs/guide/directory-structure/app-config#app-config-output tells to use declare module to extend nuxt/schema's AppConfigInput on the index.d.ts file... where is that?

I tried in src/composables/useFoo.ts instead, src/composables/index.d.ts and src/index.d.ts, but neither validates my custom entry in app.config.ts. where should this be done I? what if there are other declarations and exports on the same file?
Nuxt
Expose reactive configuration within your application with the App Config file.
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