There are a lot of categories of complicated circuit I think you can only master a couple in your lifetime. eg digital, analog, RF, metrology, high voltage and lots more and you can mix them to get more complicated.
What type of resource? A tutorial on how to build a specific complicated circuit? Or something teaching you on a progression up-to where you can design your own complicated circuits?
The latter, preferably.
Yeah, I’ve not seen anything that I love there unfortunately. Ton for the former, but not much for the later. I’ve thought of trying to create it, but I know how hard it is to produce great content and how unrewarding it is.
The datasheets for various simpler IC's often have extensive example applications with discussion of the principles. Texas Instruments' site is a goldmine.