With comments!
assets/ contains static front-end files and images. In other words, your Sass files, JS files, SVGs, or any PNGs would live here.
lib/ contains files for custom post type arguments and taxonomies. These are added to WordPress inside functions.php and could be included there, but are separated into other files to keep functions.php a bit cleaner.
views/ contains all of your Twig templates. These correspond 1 to 1 with the PHP files that make the data available to the Twig templates.