The Electron documentation covers that. In short, you have one script that the Electron executable runs as a Node script and calls the “main process” (think of this as the browser application before you’ve opened any windows) that controls all of the windows you want to open. Each window is a “renderer process” and functions very similarly to a Chrome tab.
How can I make my own css files and compile them like you can do on laravel?
You may need to be more detailed, since not everyone here has used Laravel. In Electron, you style your code the same way you do in HTML. If you want to compile Less or Sass to CSS, you can incorporate that into the build process of your application or just do it live whenever one of your files updates (you have Node available, so you can easily use packages from NPM to compile CSS).
What is the best way to store large quantities of user specific data?
It will depend heavily on your needs and use case. You can install any database you want (especially the ones with a package on NPM) or you could write to a folder on the user’s computer or make calls to a cloud service. Most storage solutions these days are designed to handle large amounts of data. If you want to make the data specific to a user account, you can put your storage in the user’s home folder.