元の質問 : How can I get the contact owners the same as the account owners?

サンプルコードがありそうで見つかりません。
仕方ない書いてみるか。

trigger AccountTrigger on Account (after update) {

    Set<Id> AccountIdSet = new Set<Id>();   
    Map<Id,Id> AccountMap = new Map<Id,Id>();
    if (Trigger.isUpdate) {
        for (Account acc : Trigger.old){
            AccountMap.put(acc.Id,acc.OwnerId);
        }
    }

    if (Trigger.isUpdate) {
        for (Account acc : Trigger.new){
            AccountIdSet.add(acc.Id);
        }
    }
	    
    List<Account> AccountList = [SELECT Id ,OwnerId, (SELECT Id,OwnerId from Contacts) FROM Account WHERE Id =:AccountIdSet];
    
    List<Contact> updateContactList = new List<Contact>();
    for (Account acc : AccountList){
        Id OwnerId = (Id)AccountMap.get(acc.Id);
        if (OwnerId != null && OwnerId != acc.OwnerId){
            if (acc.Contacts.size() >0){
                for (Contact con : acc.Contacts){
                    Contact co = new Contact();
                    co.Id = con.Id;
                    co.OwnerId = acc.OwnerId;
                    updateContactList.add(co);
                }
            }
            
        }
        if ( updateContactList.size() >0) update updateContactList;
        
    }
}

テストクラスも書いてみました。

@isTest
public class AccountTrigger_test {
    
    @isTest
    public static void test01() {
        
        Profile p;
		User u;
		p = [select Id from Profile where Name = 'Standard Platform User'];
         
		u = new User();
        u.FirstName = 'test';
		u.LastName = 'user';
		u.Alias = 'testuser';
		u.Email = 'test_user@test20220802.com';
		u.UserName = u.Email;
		u.EmailEncodingKey = 'ISO-2022-JP';
		u.TimeZoneSidKey = 'Asia/Tokyo';
		u.LocaleSidKey = 'ja_JP';
		u.LanguageLocaleKey = 'ja';
		u.profileId = p.Id;        
		insert u;
      
        Test.startTest();
        
        Account acc = new Account();
        acc.name = 'test';
        insert acc;
        
        Contact con = new Contact();
        con.LastName ='test1';
        con.AccountId = acc.Id;
        insert con;
        
        acc.OwnerId = u.Id;        
        update acc;
        
        List<Contact> ContactList = [SELECT Id ,OwnerId FROM Contact WHERE AccountId =: acc.Id];
        System.assertEquals(ContactList[0].OwnerId, u.Id);
        
        Test.stopTest();
        
    }    

}

取引先の電話を更新した際に、取引先責任者の電話にも同じものがコピーされる

public without sharing class TRG_Account {

    public static void beforeTrigger(List<Account> newList, List<Account> oldList, Boolean isInsert, Boolean isUpdate) {
    }
    public static void afterTrigger(List<Account> newList, List<Account> oldList, Boolean isInsert, Boolean isUpdate, Boolean isDelete) {
       Set<Id> AccountIdSet = new Set<Id>();   
       Map<Id,String> AccountMap = new Map<Id,string>();
       if (isUpdate) {
           for (Account acc : oldList){
               AccountMap.put(acc.Id,acc.Phone);
           }
       }
       if (isUpdate) {
           for (Account acc : newList){
             AccountIdSet.add(acc.Id);
         }
       }
       List<Account> AccountList = [SELECT Id ,Phone, (SELECT Id,Phone from Contacts) FROM Account WHERE Id =:AccountIdSet];
       List<Contact> updateContactList = new List<Contact>();
       for (Account acc : AccountList){
           String sPhone = (String)AccountMap.get(acc.Id);
           if (sPhone != null && sPhone != acc.Phone){
               if (acc.Contacts.size() >0){
                   for (Contact con : acc.Contacts){
                       Contact co = new Contact();
                       co.Id = con.Id;
                       co.Phone = acc.Phone;
                       updateContactList.add(co);
                   }
               }
           }
           if ( updateContactList.size() >0) update updateContactList;
       }
    }
}