Here is the code

Email Class

public class EmailManager {
    // Public method
    public void sendMail(String address, String subject, String body) {
        // Create an email message object
        Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
        String[] toAddresses = new String[] {address};
        mail.setToAddresses(toAddresses);
        mail.setSubject(subject);
        mail.setPlainTextBody(body);
        // Pass this email message to the built-in sendEmail method 
        // of the Messaging class
        Messaging.SendEmailResult[] results = Messaging.sendEmail(
                                 new Messaging.SingleEmailMessage[] { mail });
        
        // Call a helper method to inspect the returned results
        inspectResults(results);
    }
    
    // Helper method
    private static Boolean inspectResults(Messaging.SendEmailResult[] results) {
        Boolean sendResult = true;
        
        // sendEmail returns an array of result objects.
        // Iterate through the list to inspect results. 
        // In this class, the methods send only one email, 
        // so we should have only one result.
        for (Messaging.SendEmailResult res : results) {
            if (res.isSuccess()) {
                System.debug('Email sent successfully');
            }
            else {
                sendResult = false;
                System.debug('The following errors occurred: ' + res.getErrors());                 
            }
        }
        
        return sendResult;
    }

}

Trigger => You can use the same trigger code for the Lead object but change Opportunity to Lead wherever you can. Also I have hard coded the email address. Change that to your need.

trigger OpportunitySendEmailTrigger on Opportunity (after update) {

    List<Opportunity> opportunityList = new List<Opportunity>();

    for(Opportunity opportunity : trigger.new) {
    
        if(opportunity.StageName != trigger.oldMap.get(opportunity.Id).StageName && opportunity.StageName.contains('Closed')) {
        
            opportunityList.add(opportunity);
            
        }
    }
    
    if(opportunityList .isEmpty()) return;
    
    for(Opportunity oppRecord: opportunityList) {
    
        EmailManager emailMgr = new EmailManager();
        String sbody = 'Please use the link ' 
                    + system.URL.getSalesforceBaseUrl().toExternalForm() 
                    +'/'+oppRecord.Id + ' to go to the record!';
        emailMgr.sendMail('test@test.com', 'Opportunity ' + oppRecord.StageName, sbody);
    }

}