I hate and I love Liquid.
It is wonderful and simple, but it is a real pain since I cannot find any way to debug it or there is no advanced learning resource, and I am stuck with the few basic documentation pages by Jekyll, and the ones by Shopify are too tied to their project.
When looking at the code of Pure Liquid plugins I am amazed and I wonder how and where Allejo learnt to do so.
Can anyone on here point me to some advanced learning resources?
Debugging: I feel your pain. Do you already use the filter
inspect? Found it quite useful; or the plugin
Furthermore I learned from: Liquid-for-Programmers
Just about everything I know in Liquid from easy to hard is here
See also the Liquid guide for Jekyll without using plugins, similar to what you linked.
You can learn by reading code of others and watching tutorials or using articles or docs. But the real learning that sticks is when you build something because you need it and learn as you go how to do whatever feature of Jekyll or Liquid and apply what you find.
Memorizing syntax will only get you so far.
So I’d say set yourself a challenge to build a site for a friend or an imaginary business and go for the hard things and solve them. Or go through the whole list of Without Plugins guide I linked and build each one into your site without looking at their solution. See how far you can get alone using documentation and forums etc. And use the Without Plugins solution as a last resort or to compare when you are done.
Also another skill in general is write out code by hand on your computer without copy and pasting. Or even with the tab of original code minimized. So it forces you to both memorize the solution and also understand what it is doing, with a deeper level of attention than reading and moving on.