I need to create a trigger that can send email off of opportunity and send the template that has opportunity merge fields.I get the email from this trigger but it is missing the merge field. My trigger looks like below:

trigger ImplementationProject_EmailAlert on Opportunity (after update) {

      List<Messaging.SingleEmailMessage> mails =  new List<Messaging.SingleEmailMessage>();

    for(Opportunity newItem : trigger.new) {


        
        if (newItem.StageName == 'Closed Won ')
        system.debug('stagename@@' +newItem.stageName);
        {
            Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage(); 
//List <Contact> Con= [select id, name from Contact where name =:'Richa Paliwal']; 
            
            //system.debug('contact id@@' +Con);
            
//mail.setTargetObjectId(Con[0].id); 
mail.setSenderDisplayName('Salesforce Support'); 
mail.setUseSignature(false); 
mail.setBccSender(false); 
mail.setSaveAsActivity(false); 
EmailTemplate et=[Select id, Subject, Body from EmailTemplate where Name=:'ZSYSGEN_EXPEDITED Close Won Notification']; 
             system.debug('Template id@@' +et);
mail.setTemplateId(et.id); 
          mails.add(mail);

        }
        }

Messaging.SendEmailResult [] r = Messaging.sendEmail(mails);

}

and out of the box template looks like:
The following Opportunity includes Expedited Implementation as a product and has reached Closed Won stage: 

Date Closed Won: {!Opportunity.CloseDate} 
Opportunity: {!Opportunity.Name} 
Opportunity Type: {!Opportunity.Type} 

Account: {!Opportunity.Account} 
Agency Code: {!Opportunity.Agency_Code__c} 
Shipping State: {!Opportunity.Shipping_State__c} 

Implementation Note: {!Opportunity.Implementation_Notes__c} 

Opportunity Description: {!Opportunity.Description} 


Opportunity Link: {!Opportunity.Link}