I want to use a jQuery script on my projects template. Do I need to include the library like <script src="https://ajax.googleapis.com/ajax/libs/jquery/2.2.2/jquery.min.js"></script> in my header or is it already included by Kirbys core?
Kirbys developer put a lot of work in keeping it lightweight. Do I really need to load an external library just for one little script?
And where do I store my jQuery Code? As a .js file in the assets folder?
Plugins are extensions to the default Kirby functionality; they can in fact be anything from a file with some php helper functions, to a complete new templating engine, https://getkirby.com/docs/developer-guide/plugins
The controller folder is intended for, well, controllers, that help you to separate logic from your HTML in the templates. You should not put anything but controllers in there.