How do I find differences between 2 lists

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C#

I have two sets of lists and I wish to get the difference between the two lists but for my logic I do not seem to be getting expected results :

LIST A
A002
A75
B908
123456
672314
756213

LIST B
htg1
EDDIE1
EDDIE2
A002
A75
B908

Expected Results
To get all the new codes in List B that are not already maintained in the mapping list (List A)
This should give me all new items as below :

htg1
EDDIE1
EDDIE2

Current Output
When I apply LINQ logic for filtering I am getting all the items in List B :

htg1
EDDIE1
EDDIE2
A002
A75
B908

This is because this join query is returning 0 rows :

 List<string> joinItems = new List<string>();

joinItems = (from d1 in mappings
                         join d2 in references on d1.MappingId equals d2.CustCode
                         select d1.MappingId).ToList<string>();

Where mappings represents resultset for LIST A :

List<Partner> mappings = GetMappingsAsModel();

and references represents resultset for LIST B :

List<CustomerCode> references = GetCustomerCodes();

And to find the differences I am doing this :

List<string> cuscodes = references.Select(x => x.CustCode.ToString()).ToList();
          
           
 var newItems = cuscodes.Except(joinItems);

            
int newCodes = cuscodes.Except(joinItems).Count();

What is wrong with my Join query above ?

What I have tried:

I have tried the following alternative but still get nothing since the join query is returning nothing :

var result = references.Where(p => !mappings.All(p2 => p2.MappingId == p.CustCode)).ToString().ToList();