@Skysoft13 ejabberd.im uses erlang and XMPP protocol as their base for the implementation. It’s very much important to understand what functionalities your chat server needs and XMPP itself has a set of standard features based on the Xmpp Standards Foundation. ejabberd, mongooseim, tigase, etc., are some of the examples of open source implementations of XMPP based chat servers.

You can also implement chat servers using websockets and some example implementations are phoenix channels, socket.io (nodejs), etc.,

Not every functionality or requirements will be satisifed by the servers mentioned above, so if you need to customise you have to use erlang. elixir or any programming language the XMPP or websocket servers are built on.

The below link provides the list of standard XMPP opensource servers which adheres to the XEP standards.

Beware, each implementations have numerous configurations, which are very hard for a beginner to cope with. So, it’s better to start slow with default configurations and then get going deeper.

There are numerous opensource or GPL licensed mobile applications for using with XMPP servers.

The link has a list of supported clients, you can take a look at them and decide to use them or you can even build one from scratch ( daring move)

https://docs.ejabberd.im/developer/xmpp-clients-bots/

Checkout, the client section for the below link:
https://esl.github.io/MongooseDocs/latest/user-guide/ABCs-of-MongooseIM/