How do you implement custom Nuxt.js modules?

A custom Nuxt module is a function that receives the resolved Nuxt config and has access to Nuxt hooks and the Nuxt Kit utility library. You define it using defineNuxtModule({ setup(options, nuxt) { ... } }). Inside setup, you can use Nuxt Kit APIs: addPlugin() to inject plugins, addComponent() to register components, addImports() to add auto-imported composables, addServerHandler() to add Nitro API routes, and addTemplate() to generate virtual files. You hook into Nuxt's build lifecycle with nuxt.hook('build:before', handler). Local modules can live in the modules/ directory (auto-registered) or be published as npm packages with the nuxt-module tag.