Scenario --> I have created student VF page in which all the fields are there,So when I have inserted Id on the URL ,it's take you to the studnet record details page .when you are inserting any previous student record Id on the url of that VF page. 
But the Problem is--> As an user when the user is trying to insert or make student record on it own or new ,after hitting on the save button ,it's not taking to the studnet record details page .
Please can some one help me.

Image:-


Apex Class code-->
public class CustContStudentVfEx1ApexClass {
  
        //Now for the custom object
        //Create the visualforce Page when the user will insert the data ,it should be store somewhere so,for that we need to create
        //Variable to insert the value.
    public Student__c stud{get;set;}
     
    //Now to create the constructor when the user will insert the Id on the url we should be giving the Student Details record on the visualforce Page
    public CustContStudentVfEx1ApexClass(){
            
           //When the User will insert the Id on the url ,the particular record should be get display automatically
           ID  id=ApexPages.currentPage().getParameters().get('id');
           //Now to fetch the fields when the user will put the id on the url it should be showing some particular field on  the vf page.
           //But I want to check the user insert the url id is null or not .
           //using the conditional statement and put it inside the stud variable.
        stud=(id==null)?new Student__c():[SELECT Id,Name,State__c,Subject__c,Population__c,Gender__c,D_O_B__c,Age__c,City__c,Class__c FROM Student__c WHERE ID=:id];
            
 
    }
    //Now when the user will hit on the save button ,the record should be get shown update or new one
    public PageReference Save(){
        
             //To check the insert record is giving an error or not 
        try{
                
            upsert(stud);
        }
        catch(System.DmlException e){             
  
              //Now if an error occur I need to show this error on the vf to show an error
              ApexPages.addMessages(e);
              return null; //Now to return to the Page where we are inserting the record or fill up 
           
              
        }
        //Now ,if the error doesn't occur ,then take me to the details page of the records
        PageReference  studentDetailsPage=new ApexPages.StandardController(stud).view();
        return studentDetailsPage;
        
    }
    //For the cancel the it will take you to the current vf page
    public Pagereference Cancel(){
         
         return null;
    }
}

VisualforcePage code-->
<apex:page controller="CustContStudentVfEx1ApexClass" lightningStylesheets="true">
<apex:form >
<apex:pageBlock title="Student">
<apex:pageBlockSection title="StudentInformation"  collapsible="true" columns="2"> 

<apex:inputField value="{!stud.Name}"/>
<apex:inputField value="{!stud.State__c}"/>
<apex:inputField value="{!stud.Subject__c}"/>
<apex:inputField value="{!stud.Population__c}"/>
<apex:inputField value="{!stud.Gender__c}"/>
<apex:inputField value="{!stud.D_O_B__c}"/>
<apex:inputField value="{!stud.Age__c}"/>
<apex:inputField value="{!stud.City__c}"/>
<apex:inputField value="{!stud.Class__c}"/>\
</apex:pageBlockSection>
<apex:pageBlockButtons location="Bottom">

<apex:commandButton value="Save" action="{!Save}"/>
<apex:commandButton value="Cancel" action="{!Cancel}"/>
</apex:pageBlockButtons>

</apex:pageBlock>

</apex:form>
</apex:page>