The main Svelte site is 10/10 for me - it's really worth spending the time to go through it from front to back.
Carbon Components Svelte is an amazing set of UI components, which uses IBM's Carbon Design System. Not only does it let you put together really professional-looking UI's effortlessly (I use it for all my admin areas!), but it is really well made and documented. It's taught me a lot about how to break things up into discrete, reusable, inter-linking components.
But really, as with most things, it's setting yourself a goal and learning everything you need to achieve it. For example, I set myself the goal of creating a Virtual Tabletop (VTT). I have no need for it and no intention to release it, but I learnt so much doing it.
EDIT: Also, go and read all the Vite documentation; a lot of what I thought was SvelteKit was actually Vite, and after having spent time learning it SvelteKit is much less of a magic box to me now!