Batch Apex Class on custom object containing approval process,  to send an email to the approvers logged into the approval history related history of the custom object records ,
If at all the approval process status of the record is pending more than 24 hours
    We need 3 different email templates based on time delay, different email template should be triggered
a.    If Delayed between 24hrs and 48hrs – Email template for 24hr delay
b.    If Delayed between 48hrs and 72hrs – Email template for 48hr delay
c.    If Delayed more than 72hrs – Email template for 72hr delay
Below is the code written to achieve the above result .

global class FreightApprovalProcessBatch implements Database.Batchable<sobject>,Database.Stateful {
      
    List<Id> userList24 = new List<Id>();
    List<Id> userList48 = new List<Id>();
    List<Id> userList72 = new List<Id>();
        
    global Database.QueryLocator start(Database.BatchableContext bc)
    {        
        return 
            Database.getQueryLocator('SELECT ActorId,ElapsedTimeInHours, ProcessInstance.TargetObjectId,ProcessInstance.TargetObject.Name FROM ProcessInstanceWorkitem WHERE ElapsedTimeInHours > 24 AND ProcessInstanceId IN (SELECT Id FROM ProcessInstance WHERE Status = \'Pending\')');                         
    }   
    
    global void execute(Database.BatchableContext bc, List<ProcessInstanceWorkitem> scope)
    {        
        for(ProcessInstanceWorkitem p : scope)           
        {           
            Schema.SObjectType objectType = p.ProcessInstance.TargetObjectId.getSobjectType();
            if(objectType.getDescribe().getName().equals('Custom Object'))
            {              
                if(p.ElapsedTimeInHours >= 24 && p.ElapsedTimeInHours < 48 )
                {
                    userList24.add(p.ActorId);                 
                }
                
                else if(p.ElapsedTimeInHours >= 48 && p.ElapsedTimeInHours < 72)
                {
                    userList48.add(p.ActorId);                 
                }
                
                else if(p.ElapsedTimeInHours >= 72)
                {
                    userList72.add(p.ActorId); 
                }
            }             
        }
    }
    global void finish(Database.BatchableContext bc)
    {
        
        List <EmailTemplate> listTemplate = [Select id,name from EmailTemplate where name = 'Email Template 1' OR name = 'Email Template 2' OR name = 'Email Template 3'];   
        Id templateId24;
        Id templateId48;
        Id templateId72;
        for(EmailTemplate objEmailTemplate : listTemplate){
            
            if(objEmailTemplate.name == 'Email Template 1')
            {
                templateId24 = objEmailTemplate.Id;                
            }
            
            else if(objEmailTemplate.name == 'Email Template 2')
            {
                templateId48 = objEmailTemplate.Id;
            }
            
            else if(objEmailTemplate.name == 'Email Template 3')
            {
                templateId72 = objEmailTemplate.Id;
            }          
        }
              
        Messaging.MassEmailMessage emails24=new Messaging.MassEmailMessage(); 
        emails24.setTargetObjectIds(UserList24);       
        emails24.setTemplateID(templateId24);
        emails24.setSaveAsActivity(false);
        Messaging.SendEmail(New Messaging.MassEmailMessage[]{emails24}); 
                   
        Messaging.MassEmailMessage emails48=new Messaging.MassEmailMessage(); 
        emails48.setTargetObjectIds(UserList48);
        emails48.setTemplateID(templateId48);
        emails48.setSaveAsActivity(false);       
        Messaging.SendEmail(New Messaging.MassEmailMessage[]{emails48});
        
        Messaging.MassEmailMessage emails72=new Messaging.MassEmailMessage(); 
        emails72.setTargetObjectIds(UserList72);
        emails72.setTemplateID(templateId72);
        emails72.setSaveAsActivity(false);       
        Messaging.SendEmail(New Messaging.MassEmailMessage[]{emails72});   
    }    
}
Now, when I will run my above batch class in execute anonymous window, it is running successfully , even I am getting all those records to whom the email should be triggered.
Also, tried passing the hardcode userid's , then  I am able to receive an email .

But, when the batch is executed , I am not receiving any mails to my inbox.
So, please, let me know if some part of code is missing in the mass email messaging methods .
Also, we have created the different HTML email templates with the merge field in it, the merge fields are not populated in the received mails .
Also, let me know, how we should go and write test class for the same above class.