I have read these documentations -> websockets-next and reference guide websockets-next.

I understand how a chat message could be implemented, because the client reacts to the endpoints but I have no idea how the server could send messages to the clients if updates occure on the server.
Secondly, how can I send not only text messages but whole DTOs?

I have never worked with sockets so I only know REST-Endpoints.
My example code:

DTO for the clients:

@Data
public class InformationForClientDTO {
    private List<FinancialCalculationsDTO> list;
    private int version;
}

Code on the server:

@ApplicationScoped
public class ServerCode {

    public void someCalculation(){
        //Server has reached some point which is important for clients. Send listForClients to all connected clients:
        Arraylist<InformationForClientDTO> listForClients;
        //How to send this list to the clients? How can I call the websocket with all clients and send them the DTOs?
    }

}

Code-example From the quarkus wiki:

@WebSocket(path = "/chat/{username}")  
public class ChatWebSocket {

    // Declare the type of messages that can be sent and received
    public enum MessageType {USER_JOINED, USER_LEFT, CHAT_MESSAGE}
    public record ChatMessage(MessageType type, String from, String message) {
    }

    @Inject
    WebSocketConnection connection;  

    @OnOpen(broadcast = true)       
    public ChatMessage onOpen() {
        return new ChatMessage(MessageType.USER_JOINED, connection.pathParam("username"), null);
    }

    @OnClose                    
    public void onClose() {
        ChatMessage departure = new ChatMessage(MessageType.USER_LEFT, connection.pathParam("username"), null);
        connection.broadcast().sendTextAndAwait(departure);
    }

    @OnTextMessage(broadcast = true)  
    public ChatMessage onMessage(ChatMessage message) {
        return message;
    }

}