Some resources that might help:
JavaScript is the lowest common denominator for running in browsers so that pretty much has to be used. TypeScript is popular and so is also provided for people using that but if you’re not using it, or use some other compile-to-JS language, you can ignore those files.
Webpack is bundling all of the sources files used in development and preparing them for production. Depending on your configuration, this could include various transformations that aren’t necessary during development, or would make development too slow. It also provides a dev server so that you can iterate more quickly on your UI without having to deploy the front end to a canister after every change.