I want to think that I understand the basic game mechanics. I have no big issues with research or construction. But I still can not win wars even as the strongest powers and I have played quite a lot. How much would one have to play to understand "the basics".
Let me address a few things, as you have actually asked a complicated question in this (and other) post.
1) The game's mechanics are pretty complex. In terms of Paradox games, Vic2 is probably more complicated (or at least opaque), but in HOI4, there are plenty of small things that you may not know about influencing game play. And all of those small things tend to add up. Let me give an example: Did you know that you could have 1940 light fighters in production (as Britain) with the proper design company by late 1939? The way the NFs give bonuses stack with the design company and a few other things to result in the production of some of the best planes in the war
years ahead of time. It's like having the Mustang flying before the US enters the war. This is a
huge advantage once you know how to do it, because even if you don't have tons of air XP, the 1944 light fighter is so much better than the 1940 light fighter that you can get insane kill ratios against enemy planes. And since air power is so damn important, you do the math. In this case, a single course of action has cascading results that affect the entire war.
2) The AI is really bad at production. Not just in terms of knowing what to produce, but in terms of leaving MIC alone so that it actually builds production efficiency. I've said this before on the forum (and I stole the phrase anyway), but HOI4 is often won in the factories. Even a 5% difference in factory efficiency over the course of two years can make a huge difference. And humans that know how to run their factories can get insane advantages over the AI even when the AI should have more factories.
3) The AI poorly utilizes templates. Human templates are so much better. I can achieve wonderful concentrations of force against AI opponents just by utilizing good templates. If you get crazy and stack silly minister and general bonuses on top of odd division combinations, you can achieve even better results. :sigh: (CAV strangeness)
4) New players often don't understand width, how soft attack works, or what doctrines are actually doing. Some of this is just the game being opaque, but if a player does not understand what soft attack and hardness are, they are not going to understand how in the Hell to design divisions or counter other designs.
5) Air power is crucial, but its effects are not well understood by newer players. Hell, aircraft design is not even well understood by experienced players, because the relationship between reliability, air attack, and agility is really damn complicated. I didn't even understand it until I did hours worth of tests to figure out how to design planes.
6) The AI can be lured into bad combats. It would take too long to explain, but you can defeat the Soviet Union as Germany by just sitting in place and defending against Soviet attacks until the Soviets kill millions trying to dislodge you from your starting positions in Barbarossa.