My visualforce pag:
<apex:page standardController="Candidate__c" extensions="MyController">
    <apex:form >
        <apex:pageBlock title="Survey Form">
            <apex:pageBlockSection title="Experience Details">
                <apex:pageBlockTable style="width:165%;vertical-align:left;" value="{!expList}" var="exp">
                    <apex:column headerValue="Company Name" >
                        <apex:outputText value="{!exp.Name}"/>
                    </apex:column>
                    <apex:column headerValue="Start Date" style="width:300px">
                        <apex:outputText value="{!exp.Start_Date__c}"/>
                    </apex:column>
                    <apex:column headerValue="End Date" style="width:300px" >
                        <apex:outputText value="{!exp.End_Date__c}"/>
                    </apex:column>
                    <apex:column headerValue="Salary" >
                        <apex:outputText value="{!exp.Salary__c }"/>
                    </apex:column>   
                    <apex:column headerValue="Document Submitted?" >
                        <!--<apex:outputText value="{!exp.Documents_Submitted__c}"/>-->
                        <apex:selectradio value="{!exp.Documents_Submitted__c}" >
                             <apex:selectoptions value="{!types}"></apex:selectoptions>
                        </apex:selectradio>
                    </apex:column>              
                </apex:pageBlockTable>            
            </apex:pageBlockSection>
            <apex:pageBlockButtons location="Bottom">
                <apex:commandButton value="Submit" action="{!submit}"/>
                <apex:commandButton value="Cancel" action="{!cancel}"/>
            </apex:pageBlockButtons>             
        </apex:pageBlock>
    </apex:form>
</apex:page>
My contoller class:
public class MyController{
public List<Experiance__c> expList {get; set;}
private final Candidate__c candRec;
public MyController(ApexPages.StandardController stdController){
this.candRec=(Candidate__c)stdController.getRecord();
expList = [SELECT Id, Name, Start_Date__c, End_Date__c, Salary__c, Documents_Submitted__c FROM Experiance__c WHERE Candidate__c =: candRec.Id];
}
public List<SelectOption> getTypes(){
        Schema.sObjectType sobject_type = Experiance__c.getSObjectType();
    
        Schema.DescribeSObjectResult sobject_describe = sobject_type.getDescribe();
    
        Map<String, Schema.SObjectField> field_map = sobject_describe.fields.getMap();
      
        List<Schema.PicklistEntry> pick_list_values = field_map.get('Documents_Submitted__c').getDescribe().getPickListValues();
    
        List<selectOption> options = new List<selectOption>();
    
        for (Schema.PicklistEntry a : pick_list_values) {
            options.add(new selectOption(a.getLabel(), a.getValue()));
        }
        return options;
    }
    
public void submit(){
//update candRec;
update expList;
}
}
please could any one explain each line in gettypes() method in controller class that what will happen ineternally when every statement is executed.
Schema.sObjectType sobject_type = Experiance__c.getSObjectType();

•     what will happen internally while above line is being executed?

        Schema.DescribeSObjectResult sobject_describe = sobject_type.getDescribe();

•     what will happen internally while above line is being executed?

        Map<String, Schema.SObjectField> field_map = sobject_describe.fields.getMap();

•         what will happen internally while above line is being executed?   

        List<Schema.PicklistEntry> pick_list_values = field_map.get('Documents_Submitted__c').getDescribe().getPickListValues();

•         what will happen internally while above line is being executed?

        List<selectOption> options = new List<selectOption>();

•         what will happen internally while above line is being executed?

        for (Schema.PicklistEntry a : pick_list_values) {

•         what will happen internally while above line is being executed?

            options.add(new selectOption(a.getLabel(), a.getValue()));

•         what will happen internally while above line is being executed?

        }
        return options;