Quick note, I play most of my games with the KR mod so some things might be different from vanilla.
1. What should i build for ground units?
2. How do you rush technology?
3. HOW DO YOU DO ANYTHING NAVAL RELATED?(also is there a way to put stategic bombers and stuff on aircraft carriers?)
4. How do you get more resources? Does building infastructure increse resources like it does in hoi4?
Thks.
1. Depends on which nation you play but normally I aim to have about 70% Infantry to hold the line and 30% Tanks + Motorized to breach it. When playing a minor power that can't afford tanks, cavalry is usually the way to go, especially in areas with poor infrastructure and hard terrain like China. Special forces are more location-dependent, if you have a lot of mountainous terrains it makes sense to have mountain divisions for an additional edge. If you expect to fight amphibious battles, marines are also worth the investment. Paratroopers are the most expensive force to field in terms of the unit and aircraft cost so I'd only invest in them if you have IC to spare and a specific target, such as Britain, in mind for such a capability. The important thing is that in every given time you are building the army you need for your immediate target, and once you are attacking it, move on to the build order for the next target (basically fight today's battle and build for tomorrow's war.) Special forces are particularly worth it for manpower poor states with a lot of IC since you're getting more bang to your buck into your limited manpower.
2) If you are a major power, try to get money via tech trade with powers that aren't and won't be hostile to you and invest in research. Also, research only the things you really need which means concentrating on a narrow number of units and sticking with them, one type of fighter, one type of bomber, etc. I personally prefer to branch out if I can, but generally speaking even a great power won't be able to invest in both tactical and strategic bombers. Don't research what you won't build.
3) For a new player, I recommend sticking to the more "gamey" naval builds for now. If you have IC to spare, that means a carrier fleet in a ratio of one carrier per one destroyer escort (I like to have one or two more escorts) Make sure to fully upgrade your carriers with everything you can get for them and arm the destroyers with ASW modules. If you have some battleships at the start of the game, amass them in fleets based on firing range but only deploy them once you've smashed the enemy carrier fleet and for shore support in amphibious operations. Battleships are more exposed to air attack so I'd recommend light cruisers armed with SAAs and seaplanes for their upgrades. Battleships with 21 speed or less should get all the add-ons but armour (it slows them down a lot) while battleships with more than 21 speed should definitely invest in armour at the expense of one of the other upgrades, probably torpedos.
If you don't have the free IC for all of that at once, consider building the add-ons in serial runs along with serial runs of convoys, escorts, transports and destroyers, that said, the battleship fleets really aren't that important so you might consider just leaving them as is, just don't expect much from them if you do. If you don't have the IC for much in the way of fleet construction, consider a poor man's carrier fleet made up of light carriers and destroyers or build submarines and stack them up; not much can survive 30 submarines. The really important thing is to have at least one offensive naval fleet carrier/submarine, enough transports to move your forces and enough convoys and escorts to supply them. To fight submarines, I recommend small fleets of 3 escort carriers and 3 destroyers, they are really cheap. Battlecruisers and heavy cruisers are good at hunting surface raiders, but I've recently found fleets of 3 light carriers and 3 destroyers to be better overall.
To summarize, I like to organize my fleets like this:
A) One or two offensive fleets of fully fitted 6-8 carriers + 6-10 destroyers (ASW) or 30 submarines (torpedos)
B) Legacy formations of existing battleships/older battlecruisers + light cruisers (SAA/Seaplanes) mostly for sure bombardment.
Don't build new ones as a new player.
C) A few anti-surface raider formations of 3 light carriers with 3 destroyer escorts (ASW)
D) A few anti-submarine formations of 3 escort carriers and 3 destroyer escorts (ASW)
E) Transport fleets escorted two light carriers attached to their formation if you are having trouble.
F) Be sure to research the right doctrine, carrier doctrines for a carrier fleet, indirect approach for a submarine fleet. If you have a fleet in being researched, keep if you are going the carrier route, but discard it if you are going to build submarine formations in favour of the indirect approach.
4) For resources, it depends what you mean. For industrial capacity build factories, infrastructure mostly improves movement speed in provinces and other supply modifiers so while it's well worth it in some scenarios, it won't improve your industrial output. That said, factories in turn need energy, metal, and rare metals to function. Those resources can be traded automatically or manually with the AI players, though it is sometimes worth to plan conquests according to resource acquisition when playing resource poor countries such as Japan. The thing to remember is to build your factories roughly along your resource capacity so you don't trade away too many of your supplies or money for resources (the more supply or money you have to trade away, the more IC you need to invest in making supplies and consumer goods, the more IC you need and so on.) If you are playing a state like the US, Russia, the UK, or even Germany you should be mostly fine (Germany's biggest worry is oil) The other concern about expanding IC is how soon do you expect to fight? If you have time, then a run or two of factory construction will expand your industrial resources ahead of the war, but if you expect to fight soon then you're better of building military units. IMO, IC construction should be something done on mass though a few long term serial runs can help offset the rising costs of supply as your army grows.
5) Air Force: I really recommend you stick to tactical bombers and interceptors as a new player since everything else is more tricky to use. If you want, you can also develop escort fighters and add them to your tactical bombers, but I think the resources are better spent on more interceptors. The thing about airpower is that you are not only researching the tech for the planes but also their doctrine. Fighters share the same one, but strategic bombers, CAS, and TAC/NAVS have their own.
If you really want CAS, I tend to prefer to go with fighters instead of or in addition to interceptors because they have the range to properly escort them and the fighting capacity to fend off other fighters or interceptors. CAS, however, are all about timing your attacks and need their own doctrine tree so I don't recommend them to a new player. Tactical bombers can do most of what you need and interceptors can get you air superiority on mass and shred enemy bomber formations. Naval bombers are a good anti-fleet investment if you have spare IC as they benefit from the Tactical bomber doctrine tree.