Here s a quick snippet for you that

export default {
  async fetch(request: Request) {
    if (new URL(request).pathname === '/app') {
      const ua = request.headers.get('user-agent')
      if (ua.toLowerCase().includes('android')) {
        return Response.redirect(playstoreUrl, 301)
      } else {
        return Response.redirect(appstoreUrl, 301)
      }
    } else {
      // serve a 404, your origin, or something else
  }
}