How to make a private Chat app

Hi everyone,
if a user wants to privatly ask me a question on my app how do iadd a chat feature where i can message them privatly

pls reply fast

use firebase

how do u use firebase for that

it is a database that works online

if you have specific doubts then only we can help you

i know what firebase is, but how do i get the notifications and how do i reply to them privatly

I dont think notification will even work even if app closed and for replying make a text input and when they click send add the message you wrote from your text input to your
list as value in firebase

So how do we get the private chat done, with multiple users on the app… Thanks