Hi,
here is my apex class where I am updating county field on lead with name of custom county object based on postal code.

Apex Class:
public class LeadTriggerHandler {
    
    public LeadTriggerHandler(ApexPages.StandardController controller){
        
    }
    
    public static void beforeLeadInsertHandler(List<Lead> newLeadList){
        List<Lead> updatedLeads = new List<Lead> (); 
        for(Lead newLead:newLeadList){
            if(newLead.PostalCode!=NULL){
                updatedLeads.add(newLead);
            }
        }
        if(!updatedLeads.isEmpty()){
            updateLeadCountyOnLead(updatedLeads);
        }
    }
    
    public static void beforeLeadUpdateHandler(Map<Id, Lead> oldLeadMap, List<Lead> newLeadList){
        List<Lead> updatedLeads = new List<Lead> (); 
        for(Lead newLead:newLeadList){
            Lead oldLead = oldLeadMap.get(newLead.Id);
            if(newLead.PostalCode!=NULL&&newLead.PostalCode!=''&&newLead.PostalCode!=oldLead.PostalCode){
                updatedLeads.add(newLead);
            }
        }
        if(!updatedLeads.isEmpty()){
            updateLeadCountyOnLead(updatedLeads);
        }
        
    }
     private static void updateLeadCountyOnLead(List<Lead> leadData){
        set<string> postalCodeSet = new set<string> ();
        
        for(Lead lds:LeadData){
            if(lds.PostalCode!=NULL){
                postalCodeSet.add(lds.PostalCode);
            }
        }
        Map<String, String> postalCodeCountyIDMap = new Map<String, String>();
        if(!postalCodeSet.isEmpty()){
            for(County__c county:[SELECT ID,Name,Postal_Code__c
                                  FROM County__c
                                  WHERE Postal_Code__c IN:postalCodeSet ])
                
            {
                postalCodeCountyIDMap.put(county.Postal_Code__c, county.name);
            }
            system.debug('PostalcountyMap::'+postalCodeCountyIDMap);
            
            for(Lead lds:LeadData){
                if(!postalCodeCountyIDMap.isEmpty() && postalCodeCountyIDMap.containsKey(lds.PostalCode)){
                    lds.County__c= postalCodeCountyIDMap.get(lds.PostalCode);  
                  } else {
                    lds.County__c='';
                    }
                
            }
        }
    }
}

This is the test data factory :
@isTest
public class TestDataFactory {
    
    public static list<Lead> createLeads(Integer numLeads)
    {
        list<Lead> leads = new list<Lead>();
        for(Integer i=0;i<numLeads;i++) {
            Lead l = new Lead(LastName='TestLead' + i);
            leads.add(l);
        }
        insert leads;
        return leads;
        
    }
}

I am very new to development. Please help me on writing test class for the above apex class