I am struggling to write a test for the following class and struggling to understand Http Mocks, can anyone help write the test and explain how it works?
 

@RestResource(urlMapping='/gocardless/*')
global with sharing class GoCardlessEndpoints {

    
  /*  
   HttpPost method is used to capture a HttpPost request has been sent to our rest apex class.  
   Used to retrieve data coming in the request body and performing corressponding actions  
  */  
  @HttpPost  
   global static String doPost() {  
     /*  
       RestContext Class - Allows us to access the RestRequest and RestResponse objects in your Apex REST methods.   
       RestRequest class - Allows us to pass request data into our Apex RESTful Web service method.  
       RestResponse class - Allows us to pass or send back response data from our Apex RESTful web service method  
     */  
     //Returns the RestRequest object for our Apex REST method.  
     RestRequest request = RestContext.request;  
     //Returns the RestResponse for our Apex REST method.  
     RestResponse response = RestContext.response;  
     //Access the request body with input data coming in the JSON format  
     String jSONRequestBody=request.requestBody.toString().trim();  
     //Deserializes the input JSON string into an GoCardless_Event__c object  

      ResponseResult e = (ResponseResult)JSON.deserialize(jSONRequestBody, ResponseResult.class);
      
       for(integer i=0; i< e.events.size(); i++) {
     
           GoCardless_Event__c gcevent = new goCardless_Event__c();
           gcevent.name = e.events[i].Id;
           gcevent.created_at__c = e.events[i].created_at;
           gcevent.resource_type__c = e.events[i].resource_type;
           gcevent.action__c = e.events[i].action;
           if (e.events[i].resource_type == 'mandates') gcevent.Link__c = e.events[i].links.mandate;
           if (e.events[i].resource_type == 'payments') gcevent.Link__c = e.events[i].links.payment;
           
           gcevent.Customer_Number__c = getCustomerNo(gcevent.Link__c,gcevent.resource_type__c);
           gcevent.Customer_Email__c = getCustomerEmail(gcevent.Customer_Number__c);
           List <Account> Acc = [SELECT Id from Account where email__c = :gcevent.Customer_Email__c];
           for (integer a=0; a< Acc.size(); a++) {
               if (Acc.size()>0) gcevent.Account__c = Acc[a].Id;
           }
           insert gcevent;
       }
       return 'Done';
	}

    global Static String getCustomerNo(String objectId, String Endpoint) {
    
        
        system.debug(objectId);
        String Response;
        String Headers;
        

            string BaseURL= 'https://api-sandbox.gocardless.com/';
            string Token = 'sandbox_v5Szjx49RSC3H7-OrP5GKTM5emgYmaX9fgYTfCZR';
            string bearerToken = 'w-AINtjyy0k75CFBeBZbiA9cj4ebQuut-4--HsJp';            

        // Get the XML document from the external server
        Http http = new Http();
        
            HttpRequest req = new HttpRequest();
        	req.setEndpoint(BaseURL+Endpoint+'/'+objectId);
     		req.setHeader('access_token', 'Bearer '+Token);
        	req.setHeader('Authorization', 'Bearer '+Token);
        	req.setHeader('GoCardless-Version', '2015-07-06');
        	req.setHeader('Content-Type', 'application/json');
        	req.setHeader('Accept', 'application/json');
        	req.setMethod('GET');
        
        if (!Test.isRunningTest()) {
            HttpResponse res = http.send(req);
            String jSONResponseBody=res.getBody().trim();
            Integer strStart = jSONResponseBody.indexOf('customer":"')+11;
            Integer strend = jSONResponseBody.substring(strStart).indexOf('"');
            String CustNo = jSONResponseBody.substring(strStart, strStart+strend);
            return CustNo;
        }
        else return 'EV123';
      }    
    
    Static String getCustomerEmail(String objectId) {
    
        
        system.debug(objectId);
       String Response;
       String Headers;
       string BaseURL= 'https://api-sandbox.gocardless.com/customers/';
       string Token = 'sandbox_v5Szjx49RSC3H7-OrP5GKTM5emgYmaX9fgYTfCZR';
	   string bearerToken = 'w-AINtjyy0k75CFBeBZbiA9cj4ebQuut-4--HsJp';
        
        // Get the XML document from the external server
        Http http = new Http();
        HttpRequest req = new HttpRequest();
        req.setEndpoint(BaseURL+'/'+objectId);
     	req.setHeader('access_token', 'Bearer '+Token);
        req.setHeader('Authorization', 'Bearer '+Token);
        req.setHeader('GoCardless-Version', '2015-07-06');
        req.setHeader('Content-Type', 'application/json');
        req.setHeader('Accept', 'application/json');
        req.setMethod('GET');
        HttpResponse res = http.send(req);
    	String jSONResponseBody=res.getBody().trim();
        Integer strStart = jSONResponseBody.indexOf('email":"')+8;
        Integer strend = jSONResponseBody.substring(strStart).indexOf('"');
        String CustEmail = jSONResponseBody.substring(strStart, strStart+strend);
        return CustEmail;
      }    
    
    
    
    public class ResponseResult {
    
        public Event[] events;

    }
    public class Event {
        public String id, resource_type,action;
        public datetime created_at;
		public objLinks links;          
    }

    public class objLinks{
           string mandate;
           string payment;
        }




}