This is a stellar resource for all of the various basics, tags, filters, and objects that are available to you as a Shopify Liquid developer. Bonus - they are layed out beautifully, and with snippets of example code to help you along (or jog your memory).
Despite the short number, this official list of code snippets includes a great starting place for functionality relevant to Blog, Cart, Collections, Global, Navigation, Products, and Sections. It's a wellspring of useful, a11y friendly Liquid to help you learn various algorithms and best practices.
There are about as many sites for icons as there are icons, it's overwhelming. Cut through the chud and check out icons8. This is my go to site for all manner of icons. There is also the venerable fontawesome that can get you up and running with an incredible set of icons in minutes (and for free).