This trigger adds a related opportunity for each new or updated account if no opportunity is already associated with the account. The trigger first performs a SOQL query to get all child opportunities for the accounts that the trigger fired on. Next, the trigger iterates over the list of sObjects in Trigger.New to get each account sObject. If the account doesn’t have any related opportunity sObjects, the for loop creates one. If the trigger created any new opportunities, the final statement inserts them.

trigger AddRelatedRecord on Account(after insert, after update) {
    List<Opportunity> oppList = new List<Opportunity>();
    
    // Get the related opportunities for the accounts in this trigger
    Map<Id,Account> acctsWithOpps = new Map<Id,Account>(
        [SELECT Id,(SELECT Id FROM Opportunities) FROM Account WHERE Id IN :Trigger.New]);
    
    // Add an opportunity for each account if it doesn't already have one.
    // Iterate through each account.
    for(Account a : Trigger.New) {
        System.debug('acctsWithOpps.get(a.Id).Opportunities.size()=' + acctsWithOpps.get(a.Id).Opportunities.size());
        // Check if the account already has a related opportunity.
        if (acctsWithOpps.get(a.Id).Opportunities.size() == 0) {
            // If it doesn't, add a default opportunity
            oppList.add(new Opportunity(Name=a.Name + ' Opportunity',
                                       StageName='Prospecting',
                                       CloseDate=System.today().addMonths(1),
                                       AccountId=a.Id));
        }           
    }
    if (oppList.size() > 0) {
        insert oppList;
    }
}

The trigger you’ve added iterates over all records that are part of the trigger context—the for loop iterates over Trigger.New. However, the loop in this trigger could be more efficient. We don’t really need to access every account in this trigger context, but only a subset—the accounts without opportunities. The following trigger is more efficient because its bulkified.

trigger AddRelatedRecord on Account(after insert, after update) {
    List<Opportunity> oppList = new List<Opportunity>();
    
    // Add an opportunity for each account if it doesn't already have one.
    // Iterate over accounts that are in this trigger but that don't have opportunities.
    for (Account a : [SELECT Id,Name FROM Account
                     WHERE Id IN :Trigger.New AND
                     Id NOT IN (SELECT AccountId FROM Opportunity)]) {
        // Add a default opportunity for this account
        oppList.add(new Opportunity(Name=a.Name + ' Opportunity',
                                   StageName='Prospecting',
                                   CloseDate=System.today().addMonths(1),
                                   AccountId=a.Id)); 
    }
    
    if (oppList.size() > 0) {
        insert oppList;
    }
}