Guide to new and changed features of the mod; AKA "Now you need master degree in history, economics, programming and probably anthropology to play the game", lmao.
Ok, I will start from saying that pop promotions have been completly remodeled; they are faster but capped by literacy, literacy have been renamed (or rathen reskined) to development, development impacts many things like efficiency fo new units, prestige, demand, pop growth, and most importantly (as I said before) ratio of different pop types.
POPs have been divided into Extraction Workers > Lumpen > Seasonal Workers > Industrial Workers > Industrial Specialists, (all thosoe pops up to now gradualy promoto into eachother in this line) and (traditionaly) Bureaucrats, Teachers (clargymen), Officers, Soldiers, AND Bourgeoisie (you can notice that there is no divide between capitalists and aristocrats, it makes for a better balance, I also were lazy and reskined aristocrats to lumpen, lol). Extraction workers are work only on RGOS, seasonal workers and factory workers also can work on RGOs as well as in factories (to represent industrialisation of agriculture and mining, as well as for balance reasons), ah and there are also "lumpen" but they aren't realy that productive and are only and intermidiate point between extraction and seasonal workers (and a way to fix, or at least limit, "craftsmen assimilation exploit). Except for the lumpen each next stage of development increases it's productivity, however industrial specialists can't work in RGOs (RGOs are farms and mines BTW, if you don't know) only in factories.
As has been said development impacts everything, one of the important aspects is pop growth, however it does not gorw lineary, the pop growth modifier grows up to 30% development and starts to decline in linear manner up to 100% (but you still retain half of the bonus on 100% dev), representing demographic transition (google it if you don't know 'bout it).
Now how to grow development you may ask? "Of course investing in State Services" (reskined education budget) I answer! the profits goes to all pops (in unequal manner, which is my solution to ,so called, "liquidity problem, that vanilla and most mods have), to boost development you can also use various decisions (of which the most powerfull is collectivisation for communists, that allows you to rappidly industrialise at the cost of pop growth), social spendings, reforms (including land reforms that collectivisation is part of). Also the more development you have the slower it gets without further investments. Ah and don't drop state services budget to 0%, you will start to lose development at the rate of -4% per year, (or you can do that to drop dev and maintain sweetpoint of 30% to boost pop growth but you industrial score may collapse).
Factories, the cream de la cream of this update (right next to pop promotions and development) has been largely simplified to five goods: heavy industry, food industry, light industry, luxury industry and military industry, it mostly fixes (I hope at least, lol) vanilla problem of goods bottlenecks, resulting in economic crashes, and allows for easier calculation of profits and through this easier balancing. The base Heavy Industry production are coal and iron, for food industry, grain, for light industry it is heavy industry and so is for military industry, luxury industry needs consumer industry, in addition alll factories demand other goods to maintain production, it also is part of fix to goods bottlnecks, 'cos it seems like maintenance goods are at the bottom of demand order (so it is necessary to only focus on steady supply of iron, coal, and grain to balance things) and there is too many of those rare (or not so rare) resources to write it down (you would skip this part anyway, heh)
Migration has also been altered and balanced to (I hope) historicaly plausible levels, it is also more randomized , so USA won't steal most of immigrants; You can boost it by lowering taxes or more state investments, so that the most efficient way of creatign settler state is free market with trade protectionism on international level, free market, because of low taxes (I know mr Bourgeois that it doesn't make market free but that's how it is represented in the game), and protectionism because it leaves tarrifs as main source of income,, and it is exactly how US managed to be so successfull). All of this thanks to with countless hours of balancing pop_growth-emigration ratio). (But if you find some numbers unreasonable in late game, please inform me, I will adress it. And the same goes for dev speed.
Ideologies attraction has also been altered, but I may need more feedback on it, especialy when social-liberalism, socialism, and communism, come onto the stage.
There is a lot of underlying factors, that impact the game, and to fully understand them you may need some trial and error, or you can just read the files if you understands Paradox/Vicky2 code, but it may be complicated in some places because of it's interconnected maner.
There are probably more things I forgot about, (but it is rougly it) So let me know in the comments if you want to know about something in more detail (I may even , and probably should, update the article!).
Oh I remembered something; The first hew years may be rough for the budget but it should get better as you industrialise and economy adjusts. I will adress and rework unciv countries in later update since they haven't realy be tested out and changed that much, except now isolation policy reduces their impact on world economy by reducing both their productivity and consumption, so that no everything is produced in China; it isn't 20th/21st century, :P
[This guide has been posted, and is topical as of 17.12.2020, stable release]