Fetch Linked Contact details of Account

Question

I am writing a batch, which essentially iterates over account records. following steps.

1. iterate over account based on specific conditions
2. if the account has a linked account to it(lookup to account), then fetch the contacts details(phone, email) of that account and send to web service.
3. Also, send the account Number and Name of the 1st account(step 1). I am getting the records in debug but when fetching it I get errors. Pls suggest

logic: -

global class AccountCalloutsSOAPBatch implements Database.Batchable<sObject>, Database.AllowsCallouts{
   global Database.QueryLocator start(Database.BatchableContext BC){
   
       String query = 'SELECT Id, Owner.Email, Name, AccountNumber,Partner_Name__c FROM Account WHERE Account_for_syncing__c = true and Partner_Name__c!= null';
       return Database.getQueryLocator(query);
      
    } 
   
   global void execute(Database.BatchableContext BC, List<Account> accList){
       set<Id> accId = new Set<Id>();
       List<Contact> contactList = new List<Contact>();
       Map<Id, Contact>  merchCodeVsProductMap = new Map<Id, Contact>();
       for(Account acc : accList){
            accId.add(acc.Partner_Name__c);
       
       }
       system.debug(accId);
       contactList =[select Id,Email,AccountId, Phone, FirstName, LastName from Contact where Active__c= true and Primary_Contact__c=true and AccountId IN:accId ORDER BY LastModifiedDate DESC LIMIT 1];
       system.debug(contactList);
       for( Contact productIterrator : contactList ){
            if(!merchCodeVsProductMap.containsKey(productIterrator.AccountId)) {
                merchCodeVsProductMap.put(productIterrator.AccountId, productIterrator);
            }
        }
        system.debug(contactList);
        system.debug(merchCodeVsProductMap); 
       for(Account acc: accList){
           //need suggestion here
           //1. how to get account details from step 1
           //2. how to fetch the contacts of linked account step 2
           system.debug(merchCodeVsProductMap.values());
           system.debug(merchCodeVsProductMap);
           system.debug(acc);
           //system.debug(acc.contacts[0].Email);
           //system.debug(merchCodeVsProductMap.get(acc.Partner_Name__r.contacts[0].Email));// **getting error while fetching the contact info and account info**
       }
       
   }
   global void finish(Database.BatchableContext BC){
   }
    
}