[WX] - Need advies on possibilities

Hello,

A friend of mine has got a project in which he has to build a chatting mobile app for his client for customer support along with a web back end.

His client has around 1.45 million customers across the country. From these 50% of them are using Android based smart phones.

I am blank as to what technology should one use for build such a solution.

• Will WX do for building a mobile solution?
• What should be used for building the web back end?
• What kind of hardware should one look out for from hosting companies?
• Can we think of using Firebase and FireStore from Google for this? As per my research Firebase does not support more than 100k simultaneous users connected to it. Is this correct or am I missing something here?
• Should one adopt any third party service like Layer or Slack or PushBullet or something like this? Here my research shows that the cost of supporting such a large number of users will be quite exorbitant.
• I have also checked out a very good XAMPP solution called MongooseIM platform but I don't think we can use this if we develop the mobile app in WM or is there a solution? (http://github.com/esl/MongooseIM)

So what approach should one adopt to build a two way chatting app that will work from smart phones as well as have a web interface and that should support the load of around half a million simultaneous users. Where users will be able to message each other and also send media files to each other.

TIA