You will first need to install Rasa on a server using one of three options:
- Quick Installation (easiest - shortcut for Helm Chart Installation)
- Helm Chart Installation
- Docker Compose Installation
There are multiple frontends you can use:
- Chatbot Widget by Jitesh Gaikwad (web)
- WebApp by me (web)
- Webchat by Botfront (web)
- Chatroom by ScalableMinds (web)
- Rasa Android by Dishant Gandhi (mobile - Android)
- MobApp by me (mobile - Android)
- Rasataurants by BtLutz (mobile - iOS)
- Rasa iOS/Android by Shantanupat (React Native)
Or, of course, you can build your own one. Communicating with the Rasa server is pretty easy in itself - Just GETs and POSTs 
It can be done via anything: Web, mobile, smart car, … as long as it has access to the server and you know how to build software for it.