This plugin adds a Gutenberg block with a form available in various formats, it also adds a settings page for all forms created with the plugin and a post type (leads-form) with unique settings for every form.
Front-end stuff like compiled javascript and css etc are stored in the /public folder, don't edit those since they're compiled from scss and js files in /assets/js/ and /assets/scss/.
The plugin uses Vue, jQuery, Lottie and GSAP - all except jQuery are loaded from the modules javascript file found in /assets/js/modules/leads-form.js
To compile new js and scss simply install dependecies using npm i and then run gulp using gulp (this will watch for changes in the js/scss stored in /assets/ and compile new files on change.)
The markup for the form itself is found in /templates/blocks/leads-form/leads-form.php (it's not a twig file I know, sorry), there you will also find the data being passed from Wordpress to Vue.
The data that are posted from the form are sent via javascript (/assets/js/modules/leads-form.js) to the form-controller found in lib/controllers/leads-form-controller.php.