There's a number of good tutorials on Youtube, but in my opinion the best way to learn is to simply read the modding wiki and emulate vanilla code. The code itself is not at all hard especially on a very basic level. Also, don't get disencouraged by the ones saying HoI4 is a dying game, go have a gander at the game's Steam Charts page. The sad fact is that HoI4 is the most popular and easily moddable games that focus on this era in history; Until HoI5 or some other competitor (I'm not aware of any myself) comes along, there's no real fear of people doing something like migrating back to HoI3.
By the by, it's completely feasible to make mods from HoI4 with a pirated version of the game, the only problem is if you want to publish it on the Steam Workshop, in which case you'll need to buy the base game. Otherwise you can always release it on ModDB or some such.
Oh, and I'm not just talking out of my ass here, I was in the TNO team for a couple of years. I can shed some light on any development questions you have, starting with the RK.Moskowien and India above.
TNO had for a long time chosen to tease focus trees that were whats called "skeleton trees"; The structure of the focus tree was done, but the focuses were dead, they had no content. You may have noticed that practically all focus trees teased more than a year before release never found their way ingame, because once someone got to work on them, they just redid the whole thing, or like in the case of RK.Moskowien, nobody ever got around to working on it. Matter of fact, there was at one point going to be development on RK.Moskowien, and I believe it was Panzer who began planning a "resettlement" mechanic. This raised a storm within the team as many did not agree with including what was essentially a genocide mechanic in the game, and in the end, Panzer was shot down by most of the team, and RK.Moskowien was dead in the water.
India is a different story. India's main developer, a person called Mupper, was kicked off from the team towards the end of development for the release build. Well, she (Pretty sure Mupper's a woman, but I've been burned before) pretty much signed her own death penalty in the end. You see, as some of you have already discovered, India was practically code-complete like half a year before release, I believe it was the first nation to be code-complete in fact. However, now we get to the reason India was cut; The code, for the most part, sucked dick, and there was practically no loc (Localization, the written text you see) work done for the nations. So, upon finally discovering this (TNO always had very weak quality control in place), the team decided to shelf India for post-release content, as it would simply be impossible to finish it in time for release. Well, naturally Mupper did not like seeing her baby getting effectively killed in front of her, so when the higher-ups of the dev team were telling her this, she effectively told them to eat shit, and she was gone from the team.
Many such cases! Another nation to face an almost similar fate was Japan. The Japan team had a number of team leaders during its development, most famously a person known as Morrina. Afterwards, many concluded he was in over his head, but it is what it is. Morrina very nearly crashed Japan completely, almost sharing India's fate, but this wasn't a matter of quality, it was a matter of lying. You see, team leaders were obligated to keep the main developers up-to-date on how their departments were progressing. Japan wasn't doing that hot, but Morrina chose to not enlighten the developers. So, when Japan was eventually reaching code-completion, someone noticed that not all was well with Japan; Once again, it was extremely lacking in the loc department, something you can see to this day in Japan's events. Morrina was questioned, the team was questioned, Morrina was removed, and soon this small bit of developer drama turned into personal drama as Morrina began circulating a claim that Panzer had blackmailed him into working on the mod, even though he did not want to. This was fun and all to follow, but in the end it turned out Morrina was once again lying.
The "shadow council" thing where a number of team member made a secret Discord server to act behind Panzer's back has been brought up here before; If memory serves, and I'm 95% sure it does, Morrina was the one who couldn't take the secrecy and ratted the server out to Panzer, managing to escape without punishment. This all happened months before he was fired of course.
Quite the wall of text, I know, but if you'd like to know more, feel free to ask.
