HI Punit,
This should help answer your query:
https://salesforce.stackexchange.com/questions/184525/help-me-to-undestand-this-lightning-helper-methods-reduce-showhelpmessageifin

Copying inputs from above link:

showHelpMessageIfInvalid - what this will do? Do we have some other methods like these? where we can see these documentation?

This method is used by Lightning:Input (https://developer.salesforce.com/docs/component-library/bundle/lightning:input/specification)tag to check for validity of input tag.

showHelpMessageIfInvalid(): Shows the help message if the form control is in an invalid state.
Lets take this input example:

   <lightning:input aura:id="field" label="Last name" placeholder="Last name" required="true" /> 
since required is marked true if user doesn't enter any value the validity object for this input control will have value as valueMissing.

The other possible values are :

• badInput: Indicates that the value is invalid

• patternMismatch: Indicates that the value doesn't match the specified pattern

• rangeOverflow: Indicates that the value is greater than the specified max attribute

• rangeUnderflow: Indicates that the value is less than the specified min attribute

• stepMismatch: Indicates that the value doesn't match the specified step attribute

• tooLong: Indicates that the value exceeds the specified maxlength attribute

• typeMismatch: Indicates that the value doesn't match the required syntax for an email or url input type

• valid: Indicates that the value is valid

• valueMissing: Indicates that an empty value is provided when required attribute is set to true

when a form is submitted or field validity code is triggered. Below javascript code which is present in the same link runs and checks for validity of input components

({
    onClick: function (cmp, evt, helper) {
        var allValid = cmp.find('field').reduce(function (validSoFar, inputCmp) {
            inputCmp.showHelpMessageIfInvalid();
            return validSoFar && inputCmp.get('v.validity').valid;
         }, true);
         if (allValid) {
             alert('All form entries look valid. Ready to submit!');
         } else {
             alert('Please update the invalid form entries and try again.');
         }
     }
})
If you look at the above function reduce function is used again. just to break it down into smaller chunks to explain in detail

Lets say you have 4 lightning:input components with id as 'field', component.find will return an array of 4 elements

Array.reduce will run for each element and the function inputCmp.showHelpMessageIfInvalid(); will check if the element is valid if not it will show the help message and the red border to highlight the field is invalid.

The third line 'return validSoFar && inputCmp.get('v.validity').valid;' is basically like an if and condition ,so in this case which is your third question(' v.validity').valid checks if the component is in a valid state. v.validity is part of every lightning:input component to check for validity of the input component. Lets say that a lightning:input is marked required and user has not entered a value when this code runs it will return false which basically means the component is not valid.

The return statement will return false even if one form component of your array fails for any of the invalidity status and overall allValid variable will become false.

Finally the allValid will be false and an alert message will show in UI which indicates one more lightning:input tags are in invalid state. It means they have not entered data or they have not respected the fields formatter attribute etc.

​​​​​​​Hope this helps you. Please mark this answer as best so that others facing the same issue will find this information useful. Thank you