Hi Shruthi,

Please check the below code:
 

global class SendNotificationBatch implements Database.Batchable<sObject>, Schedulable, Database.Stateful {

    //Variable Section
    global FINAL String strQuery;
    global List<String> errorMessages = new List<String>();
    
    global SendNotificationBatch() { 
        this.strQuery = getBatchQuery();
    }
    
    //Returns the Query String to Batch constructor to fetch right records.
    private String getBatchQuery() {
        String strQuery = 'SELECT Id, CloseDate, Owner.Email FROM Opportunity WHERE CloseDate < TODAY'; 
        return strQuery;
    }
    
    //Batch Start method
    global Database.QueryLocator start(Database.BatchableContext BC) {
        return Database.getQueryLocator(strQuery);
    }

    //Batch Execute method calls findCostForWoD method
    global void execute(Database.BatchableContext BC, List<sObject> scopeList) {
        System.debug(LoggingLevel.INFO, '== scopeList size ==' + scopeList.size());
        
        List<Opportunity> oppList = (List<Opportunity>) scopeList;
        if(!oppList.isEmpty()) { 
            List<Messaging.SingleEmailMessage> mailList = new List<Messaging.SingleEmailMessage>();
            for (Opportunity prod : oppList)
            {               
                
                Messaging.SingleEmailMessage message = new Messaging.SingleEmailMessage(); 
                String[] toAddresses = new String[] {prod.Owner.Email};
                Message.setToAddresses(toAddresses); 
                Message.SaveAsActivity = false;
                mailList.add(Message);
                
            }
            if(!mailList.isEmpty()) {
                try{
                    Messaging.sendEmail(mailList);
                }
                catch (Exception ex) {
                    errorMessages.add('Unable to send email to Tech: '+ ex.getStackTraceString());
                }
            }
        }
    }  

    //Batch Finish method for after execution of batch work
    global void finish(Database.BatchableContext BC) { 
        AsyncApexJob aaj = [Select Id, Status, NumberOfErrors, JobItemsProcessed, MethodName, TotalJobItems, CreatedBy.Email from AsyncApexJob where Id =:BC.getJobId()];
        
        // Send an email to the Apex job's submitter notifying of job completion.
        Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
        String[] toAddresses = new String[] {aaj.CreatedBy.Email};
        mail.setToAddresses(toAddresses);
        mail.setSubject('JOB Salesforce Send Notification Batch: ' + aaj.Status);
        String bodyText='Total Job Items ' + aaj.TotalJobItems + ' Number of records processed ' + aaj.JobItemsProcessed + ' with '+ aaj.NumberOfErrors + ' failures.\n';
        bodyText += 'Number of Error Messages ' + errorMessages.size() + '\n';
        bodyText += 'Error Message' + String.join(errorMessages, '\n');
        mail.setPlainTextBody(bodyText);
        Messaging.sendEmail(new Messaging.SingleEmailMessage[] { mail });
    }
    
    //Method which schedules the ProductDownloadBatch
    global void execute(SchedulableContext sc) {        
        SendNotificationBatch snInstance = new SendNotificationBatch();
        ID batchprocessid = Database.executeBatch(snInstance);
    }
}

Batch Apex
A Batch class allows you to define a single job that can be broken up into manageable chunks that will be processed separately.

When to use Batch Apex
One example is if you need to make a field update to every Account in your organization. If you have 10,001 Account records in your org, this is impossible without some way of breaking it up. So in the start() method, you define the query you're going to use in this batch context: 'select Id from Account'. Then the execute() method runs, but only receives a relatively short list of records (default 200). Within the execute(), everything runs in its own transactional context, which means almost all of the governor limits only apply to that block. Thus each time execute() is run, you are allowed 150 queries and 50,000 DML rows and so on. When that execute() is complete, a new one is instantiated with the next group of 200 Accounts, with a brand new set of governor limits. Finally the finish() method wraps up any loose ends as necessary, like sending a status email.

Sample Batch Apex
1) Start Method is automatically called at the beginning of the apex job. This method will collect record or objects on which the operation should be performed. These record are divided into subtasks & passes those to execute method.

2) Execute Method performs operation which we want to perform on the records fetched from start method.

3) Finish Method executes after all batches are processed. Use this method to send confirmation email notifications.

When to use Batch Apex
One example is if you need to make a field update to every Account in your organization. If you have 10,001 Account records in your org, this is impossible without some way of breaking it up. So in the start() method, you define the query you're going to use in this batch context: 'select Id from Account'. Then the execute() method runs, but only receives a relatively short list of records (default 200). Within the execute(), everything runs in its own transactional context, which means almost all of the governor limits only apply to that block. Thus each time execute() is run, you are allowed 150 queries and 50,000 DML rows and so on. When that execute() is complete, a new one is instantiated with the next group of 200 Accounts, with a brand new set of governor limits. Finally the finish() method wraps up any loose ends as necessary, like sending a status email.

Limits in Batch Apex
1) Up to five queued or active batch jobs are allowed for Apex
2) Cursor limits for different Force.com features are tracked separately. For example, you can have 50 Apex query cursors, 50 batch cursors, and 50 Visualforce cursors open at the same time.
3) A maximum of 50 million records can be returned in the Database.QueryLocator object. If more than 50 million records are returned, the batch job is immediately terminated and marked as Failed
4) If the start method returns a QueryLocator, the optional scope parameter of Database.executeBatch can have a maximum value of 2,000. If set to a higher value, Salesforce chunks the records returned by the QueryLocator into smaller batches of up to 2,000 records. If the start method returns an iterable, the scope parameter value has no upper limit; however, if you use a very high number, you may run into other limits.
5) If no size is specified with the optional scope parameter of Database.executeBatch, Salesforce chunks the records returned by the start method into batches of 200, and then passes each batch to the execute method.Apex governor limits are reset for each execution of execute.
6) The start, execute, and finish methods can implement up to 10 callouts each
7) The maximum number of batch executions is 250,000 per 24 hours
8) Only one batch Apex job's start method can run at a time in an organization. Batch jobs that haven’t started yet remain in the queue until they're started. Note that this limit doesn’t cause any batch job to fail and execute methods of batch Apex jobs still run in parallel if more than one job is running

Need of Batch Apex: - As you all might know about the salesforce governor limits on its data. When you want to fetch thousands of records or fire DML on thousands of rows on objects it is very complex in salesforce and it does not allow you to operate on more than certain number of records which satisfies the Governor limits.
But for medium to large enterprises, it is essential to manage thousands of records every day. Adding/editing/deleting them when needed.
Salesforce has come up with a powerful concept called Batch Apex. Batch Apex allows you to handle more number of records and manipulate them by using a specific syntax.

1) When you want to process large number of records on daily basis or even on specific time of interval then you could go for Batch Apex
2) Also, when you want an operation to be asynchronous then you could implement the Batch Apex. Batch Apex is exposed as an interface that must be implemented by the developer. Batch jobs can be programmatically invoked at runtime using Apex. Batch Apex operates over small batches of records, covering your entire record set and breaking the processing down to manageable chunks of data

Please go through the below links:

https://developer.salesforce.com/forums/ForumsMain?id=906F0000000DBtv

Please do let me know if it helps you.

Regards,
Mahesh