https://join-lemmy.org/docs/contributors/02-local-development.html#frontend-development
That should get you running locally. If you’re familiar with React, you should feel right at home. In either case, the Inferno docs should help you out a lot.
You’re correct that it’s transpiled into minified JS, but the source is in human-friendly TypeScript. The transpilation is handled automatically for you by Yarn.