Create one Object Reporting country . Create 3 fields country_code,country,capital. In object Account create field country_code. Now Create Lookup to Reporting country in Opportunity. Lets take example one record of Object Reporting country country_code country capital 91 India Delhi if In Account Object country_code = 91 then all opportunities …

opportunity_final_trigger_Handler.apex

public class Testhandler {
    public static void handleTrigger (Map<Id, Account> newVal) {
        
        List<Opportunity> listToUpdate = new List<Opportunity> ();
          
  		Map<Decimal, Reporting_country__c> ctrMap = getCountryData (newVal);
      
        for (Account ac : getRelatedOpportunity (newVal).values()) {
            for (Opportunity opp : ac.Opportunities) {
                if (ctrMap.containsKey(ac.country_code__c)) {
                 	opp.reporting_country__c = ctrMap.get (ac.country_code__c).Id;
                	listToUpdate.add (opp);   
                }
            }
        }
        if (listToUpdate.size() > 0) {
         	update listToUpdate;   
        }
    }
    
    private static Map<Id, Account> getRelatedOpportunity (Map<Id, Account> newVal) {
       return new Map<Id, Account> ([SELECT Id, country_code__c,(SELECt id, reporting_country__c from Opportunities) from Account where id in : newVal.keySet()]);
    }
    
    private static Map<Decimal, Reporting_country__c> getCountryData (Map<Id, Account> newVal) {
        Map<Decimal, Reporting_country__c> ctrMap = new Map <Decimal, Reporting_country__c> ();
        Set<Decimal> cCode = new set<Decimal> ();
        
        for (Account a : newVal.values()) {
            cCode.add (a.country_code__c);
        }
        for (Reporting_country__c  ctr : [Select country_code__c from Reporting_country__c where Country_Code__c in : cCode] ) {
            ctrMap.put(ctr.country_code__c, ctr);
        }     
        return ctrMap;
    }
}

OpportunityAssignment_rudimmentry.apxc

public class Testhandler {
    public static void handleTrigger (Map<Id, Account> newVal) {
        
        Set<Decimal> cCode = new set<Decimal> ();
        Map<Decimal, Reporting_country__c> ctrMap = new Map <Decimal, Reporting_country__c> ();
        List<Opportunity> listToUpdate = new List<Opportunity> ();
          
        for (Account a : newVal.values()) {
            cCode.add (a.country_code__c);
        }
        
        System.debug('Country Code : ' + cCode );
        for (Reporting_country__c  ctr : [Select country_code__c from Reporting_country__c where Country_Code__c in : cCode] ) {
            ctrMap.put(ctr.country_code__c, ctr);
        }   

       	Map<Id, Account> relatedOpportunity = new Map<Id, Account> ([SELECT Id, country_code__c,(SELECt id, reporting_country__c from Opportunities) from Account where id in : newVal.keySet()]);

        System.debug('CtrMap : '+ ctrMap );
        for (Account ac : relatedOpportunity.values()) {
            for (Opportunity opp : ac.Opportunities) {
                System.debug('ac.Opportunities : '+opp);
                
                System.debug('Code :' + ctrMap.get(ac.country_code__c));
                opp.reporting_country__c = ctrMap.get (ac.country_code__c).Id;
                listToUpdate.add (opp);
            }
        }
        System.debug(listToUpdate);
        update listToUpdate;
    }
}

trigger_opp_Account.apex

trigger AssignmentSecondTrigger on Account (before update, after update) {
    if (Trigger.isUpdate) {
        if (Trigger.isBefore) {
            System.debug('*****Before Update');
        }
        if (Trigger.isAfter) {
            System.debug('*****After Update');
            Testhandler.handleTrigger (Trigger.newMap);
        }
    }
}