In which folder of an extension are the CSS, JavaScript and image files located that the web server should deliver?

Publicly accessible files of an extension belong in Resources/Public/, for example Resources/Public/Css/main.css or Resources/Public/Icons/Extension.svg. In a template you include them via paths like EXT:my_ext/Resources/Public/Css/main.css, and TYPO3 generates the public URL from it. Resources/Private/ is the obvious mistake: it holds templates, partials, layouts and language files that are only read on the server and should explicitly not be accessible from outside. Files created at runtime do not belong in the extension but in typo3temp/.