I am absolutely new with Magento. Have basic knowledge with custom modules and front-end layout updates.
I need to insert a custom validation for the ZIP Code in my checkout page (One-page checkout). Zip codes are from database, there is a custom admin module for the business administrator who can put insert/update ZIP codes with it's respective Country Code (Both are separate fields).
I have tried to customize "/view/frontend/layout/checkout_index_index.xml" with no luck. Most of the examples are with JS solution. Some of them with event observer and custom modules but not clear enough. Unable to figure it out how to validate the data with the inserted "ZIP Code" and selected Country.
Any support or proper direction will much appreciated.