1. Wild horror - a mass of plug-ins (often of dubious quality). And the scripts and styles from them - this is a consequence, a side effect.
2. Scripts and styles are connected by the theme of functions.php (see answer @ch3rri) + by plugins. Scripts/styles are connected as external files using wp_enqueue_script/style and inline through wp_add_inline_script/style. And can transmit data via wp_localize_script.
3. Disabling scripts and styles is done through wp_dequeue_script/style
4. If by "clean up" you mean delete them all, then delete the plugins themselves, otherwise it will break their functionality
5. If by "clean" you mean a concatenation of scripts and styles in one (or more) files, see caching plugins and, in fact, a concatenation of: