Vehicle Selector Dropdown Integration

I’d like to add a three-level “Make / Model / Year” dropdown to my WordPress site, styled and working just like the one on https://autokeypro.ca/. My site runs the Newspaper theme with no page-builder, so the solution should fit cleanly into a standard template or a dedicated shortcode/block.

Scope
• Build or configure a lightweight WordPress component—custom plugin, child-theme snippet, or JavaScript module—that shows the cascading dropdowns.
• Populate the lists with a starter data file (CSV, JSON, or array) that I can easily update later. No need to hook into an existing database or plugin.
• Ensure the selector is responsive, inherits my theme’s fonts/colours, and loads quickly.
• Provide a simple way for me to determine what happens after the user makes a selection (e.g., redirect to a URL pattern, submit a form, or trigger JS)—I’m open to your suggestion.

Deliverables
1. Working dropdown integrated on a test page of my site.
2. Source files (PHP, JS, CSS, data file).
3. Clear instructions for future data edits and, if applicable, plugin installation.

Please share a brief outline of how you’d implement it and any similar WordPress selectors you’ve built.