I wrote a trigger on lead object where it Inserts opportunity record and update the Lookup field on Lead with Opportunity ID.

Here is my code it is inserting Opportuinity Record but it is not updating the field. And I want to use after Update only !! 
 

public class TriggerHelper {
 
    public static boolean isExecuting = false;
   
 
    public static void updateValues(){
  List<Opportunity> opp = new List<Opportunity>();
        if( TriggerHelper.isExecuting ){
            // if was executed durinListg the same context 
            // avoid recursion
            return;
        }
 
        TriggerHelper.isExecuting = true; 
        
        //list of new instance of object to update. 
        Lead[] lds = new Lead[]{};
      System.debug('@@@@@@@@@@@@@@@@@@@@@@@@@@@@');
        for (Lead a : (List<Lead>)Trigger.new) {  
        if(a.LastName == 'test'){
    
    Opportunity op = new Opportunity();
    op.Name = a.LastName;
    op.CloseDate= System.today()+20;
    op.StageName = 'Prospecting';
    opp.add(op);
    
    System.debug('$$$$$$$$$$$$$$$$$$$$$$$$$$$$$'+op);

            //here the magic by creating a new instance using the same id
            Lead aux  = new Lead(Id = a.Id);       
            aux.Opportunity__c =  opp[0].Id;
            
     
        System.debug('%%%%%%%%%%%%%%%%%%%%%%');
        } 
        
    }   
        //then update the new instance list. 
        Insert opp;
        Update lds;
 
    }
 
}