How to expose restapi class to the Legacy systems ?

By using @RestResource class and annotations like @httppost, @httpget ,@httpput, @httpdelete..etc

we need to place these annotations before the Methods which we want to expose..

ex:  @RestResource(urlMapping='/Merchandise/*')

global with sharing class MerchandiseManager {
  
    @HttpGet
    global static Merchandise__c getMerchandiseById() {
        RestRequest req = RestContext.request;        
        String merchId = req.requestURI.substring(
                                  req.requestURI.lastIndexOf('/')+1);
        Merchandise__c result = 
                       [SELECT Name,Description__c,Price__c,Total_Inventory__c
                        FROM Merchandise__c 
                        WHERE Id = :merchId];
        return result;
    }
  
    @HttpPost
    global static String createMerchandise(String name,
        String description, Decimal price, Double inventory) {
        Merchandise__c m = new Merchandise__c(
            Name=name,
            Description__c=description,
            Price__c=price,
            Total_Inventory__c=inventory);
        insert m;
        return m.Id;
    }
}