I’m just thinking of building an online chat system. It’s like a clone version of discord but in my way and I don’t really have that knowledge about creating chat systems.
My question is how chat systems are made (I did use google and I didn’t find anything useful) i need someone to explain how a chat system works

With php you have to use an pull based system Like @Demolishun proposed. But i wouldnt delete the messages.

A lot of chat systems are either using or inspired by IRC/XMPP, both of which are publically documented (more or less successfully, since implementation details differ) and have multiple open source servers and clients that you can inspect if you want to see how actual systems are doing it. (IRCv3 is doing some cool stuff, IMO.) Then, you can take what you've learned and do what you want with it.

@Demolishun i did that too, but it was very amateur.i did send data using ajax to php server and then save the data in a database and get back the data and stores it in a json file and then read it via js and i made the set-interval method with 1 second gap. Then display it on the web. But didn’t work as it should be