I know the basics and most of the aspects but the parts that really confuse me is Construction i.e (What to bulid and when do you bulid?) Research i.e (What to research in general and in what order?)
unfortunately this is super situational, for instance if you are a nation with a VERY small industrial base or is going to a very large early war(for instance, China) its better to just ignore civs and go mils
however, a general rule is that if you have the option, a half-decent base industry, and are strong enough to handily win any early wars that you have planned without military industry expansion go to about '38(where specifically is up to you) and then from there start militarizing unless you are a big exception and aren't going to start the war or be dragged into it by '39 or '40. This is so you have a healthy civ base to expand during the war and repair any potential damage the enemy might do(for instance compared to a nation that just goes mils, you'll be weaker early in the war but a nation that neglects its civ base will have trouble sustaining themselves against a more balanced economy)
Focus trees i.e (What to focus on and when?)
now this is the king of being situational, considering that any case that isn't generic will be different in its own ways. but if you want my advice, I'll give some for Generic Focus Tree:
Do your industry before you do conquering. This is because industry(and also the Naval Focus) has a tendency to place its factories on non-core land, and that's just unfortunate since you won't be able to reliably use those factories from those foci
Fascism, in spite of the new Army Spirit available to Communism, is the best way to go if you are going raw power, as the additional 7.5% Recruitable Population is incredibly valuable as long as your nation has any sort of base population to work with
Also, Remember that Many Minor Nations have formable nations that is incredibly valuable for those nations being able to sustain their punching power, while others are just end-game goals(for instance, Great Zimbabwe is practically needed for South Africa to boot the UK out of Africa, while the Greater German Reich, Arabia, or Persia have you practically already won by the time you formed them)
production(How many mils do you add on … and how do you decide what to add the mills on?) And lastly, how do you determine your industrial capabilities to bulid tanks? Thank you.
Depends on the nation that you are playing and the war that you are expecting
this is entirely a question about when to refine your warfare. There's no concrete answer for what point you should do what, just that you should ALWAYS prioritize your basics-making sure you have Infantry and Artillery to fill the line-if you don't have this, then there's nothing for the refinements to be based off of to improve your war capabilities
once you have the basics(Inf. Equipment, Arty, AA, AT, and Sup. Equipment) fufilled and done well, then you can start refining which depends on what wars you are going to fight(Flat terrain and decent supplies? Tonk Time, Rough Mountains and Hills? Get Mountaineers up and running. Enemy has little airpower? Start some airpower in the forms of flexible fighters and CAS. Enemy has lots of airpower? Focus on Fighters to prevent the enemy planes from screwing you over) as for the Tank Question, LTs with Infantry Guns are a good way of getting very punchy tanks for relatively cheap and are a good pick for a nation wanting to expand what it has for breaking through, furthermore until you get to about '38 or so and start getting medium cannons there really isn't a reason or rush to go Medium Tanks unless you value a specific armor value that is highly important. Generally though if you use them sparingly a couple of Mils should be enough for a relatively small, but still plenty potent tank force to do offensives with, just know what your limits are with using them and don't risk them too much with bad attacks or unnecessarily against or around enemy tanks(Tanks are some of the best things to counter Tanks, but because you are new if you haphazardly do an offensive around enemy tanks then there's a real risk of your tanks getting encircled)