If you have plugins that need to access $auth, you can use auth.plugins option.

nuxt.config.ts
{
    modules: [
        '@nuxt-alt/auth'
    ],
    auth: {
        plugins: [
            '~/auth-plugins/auth.ts' 
        ]
    }
}
auth-plugins/auth.ts
export default defineNuxtPlugin(({ $auth }) => {
    // useAuth() is preferrable
    if (!$auth.loggedIn) {
        return
    }

    const username = $auth.user.username
})