How do I implement a push server?

Hi, everyone. I've never made a chat app before, so I'm making a chat app. It's a simple text transmission base. If the other person sends me a text message, it has to pop up on my phone right away. I had to have a push concept to implement this, so I thought of a web socket. The old Kakao Talk uses a polling method that asks the server once every 10 minutes, but these days, the level seems to be low, so I think we need a way to keep the connection continuously. But all I can think of is a web socket. At one time, it was supposed to designate a push server when developing an iPhone, but now I don't remember because I have a vague memory. Anyway, I think there are several other ways, but I'm not sure. May I ask what methods there are? Thank you.