Problem statement:

from Items object (related object on oppty) copy type__c values and Amount__c values and create (only insert) records on object item_product__c (related object on oppty)after calulating amount__c and type__c.

type__c  value from Items object
A
B
C
D

if records is getting created on items object 

type__c=A   Amount__c=5
type__c=A   Amount__c=6
type__c=C   Amount__c=10
type__c=c   Amount__c=6
type__c=B   Amount__c=8
.....

records should be created on item_product__c like below

type__c=A   Amount__c=11   //(5+6)
type__c=B   Amount__c=8
type__c=C   Amount__c=16

total 3 records should get created.

Note : amount is formula field on item but same amount is currecy field on item_product__c


Createrecord(itemIdFrompreviousmethodIds); // taking id from previous method
}
 public static void Createrecord(Set<Id> itemIdFrompreviousmethodIds){
        Set<String> Types = new Set<String>();
        for(Type__mdt TypeCM : [Select type__c from Type__mdt]){ //>>>>>>>>>>> metadata where store all type__c values.
            Types.add(TypeCM.type__c);                // adding all type__c values and keeping it into set
        }
        
        
 Map<Id,List<items__c>> opptyitemList = new Map<Id,List<items__c>>();
        
         for(items__c item : [select Id,type__c,Amount__c,Opportunity__c from items__c where Id IN : itemIdFrompreviousmethodIds and product_type__c IN :Types]){    
                                  // getting all records from item which is related object of oppty 
            
            
            //mapping for item with opptyId.
             if(opptyitemList.containsKey(item.id)){
             
                  list<items__c> LineItemLst=opptyitemList.get(item.id);
                   LineItemLst.add(item);
                    opptyitemList.put(item.Opportunity__c,LineItemLst); // not getting any values.
                     
             } else { 
                 opptyitemList.put(item.Opportunity__c, new List<items__c>());  
                  opptyitemList.get(item.Opportunity__c).add(item);   // creating records and getting mapped value.
                  
                  //how should I write code so that I can create records on item_product__c 
                
                         
                     }
                     
                 }
          
             } 
             
   Any modification or any other suggestions will be appreciated.