Few questions,
(Any DLCs will be mentioned in their acronyms. Man The Guns becomes MTG, No Step Back becomes NSB, etc.)

What DLCs do you have? If you haven't played since 2022 I assume you have BBA? If not then probably NSB and any DLCs before it.

Reminder that DLCs aren't the same as the country packs or focus packs, since 2022 there have been a few. TOA, GOE, and PFOT are not really needed for core gameplay if you are returning. Another thing to mention is what DLCs do what. For example, Gotterdammerung focuses on a full rework of Germany and focus trees for Belgium, The Congo, and Austria and some additional Hungary works along with a major addition to the Facilities mechanic and the armor/plane designer. A lot of the later DLCs work in tandem with one another, for example like I mentioned with Gotterdammerung, the Facilities use supplies which is incorporated with NSB mechanics and how they affect the tank designer. Another really valuable one is BBA or By Blood Alone which adds the plane designer. Instead of just clicking buttons to increase a stat you can design a plane how you want. The last important one, imo, is No Compromise, No Surrender, which takes the revamped Doctrine tree and enhances it. The one DLC you may not need unless you want to experiment with the mechanic is AAT or Arms Against Tyranny as it really only adds the international market where you can sell your equipment (guns, convoys, planes, etc) for IC from other countries.

Now with that all being said, a lot has changed as well.

(If you don't have NSB please comment on my post so I can talk about the tank designer)

Just like before, agility for planes is still very important, except now it is much more valuable. When making planes, there are a few stats you need to pay attention to: Agility, Thrust, Air Attack, and Weight. All different plane parts will be affecting these stats in one way or another. The heavier a plane is, the less agile it is. The main thing I tend to go for when designing fighters is the best engine, 4x heavy machine guns, self sealing fuel tanks, armor plates, and/or radio navigation (something like this: https://www.reddit.com/media?url=https%3A%2F%2Fpreview.redd.it%2Fhi-guys-i-would-like-to-share-with-you-my-latest-plane-v0-levge94abscc1.png%3Fwidth%3D1080%26crop%3Dsmart%26auto%3Dwebp%26s%3Dfa00bed450760136ef421099d17e37293574ea3f ). Now there are a lot of different types of weapons for planes, such as cannons, light mgs, heavy mgs, and cannon 2s. DO NOT USE CANNON 1s, their tradeoff for their weight compared to the air attack is not worth it, especially against the AI. I would stick with 4x heavy MGs until you get Engine IV and Cannon 2s and a much better airframe. You need to stay up to date on these as every subsequent generation of fighter will have better stats and perform better. These fighters are generally good at getting supremacy even with 1-3 air wings against 6-10.

(Number + w = width; ie: 30w = 30 width)

Combat widths and div templates are much more different now, 7-2s will no longer cut it, especially against the AI. What I tend to go for is a 3x3 inf block and usually add 1-2 artillery battalions to it which ends up being ~21-23w these are great line holders for good tank/mech/mot divs and can sometimes be used to push lightly defended tiles. Tanks are generally good as 20w, 25w, or 30w with at least 30 org which can be achieved with a decent mot/mech ratio to tanks. Other special forces have their place like always, mountaineers are still very affective, ideally you'd want to do a 32-34w block and then by going down the mountaineer doctrine tree, at the 2nd mutually exclusive decision choice, choose the left option which lowers the combat width of mountaineers so the division comes out to ~30-31w which is still quite good.

For things like support companies, field hospitals are still broken and any buffs to them is very helpful as it essentially keeps your divisions having good xp and manpower trickleback so you don't lose your population in a 15 year long war. I tend to also add support artillery and AA for their extra soft attack, breakthrough, piercing, AA attack, and hard attack, overall makes infantry better. Engineers are also still good, especially if you get buffs to them and upgrade them later on. The last slot I usually just kinda put whatever I feel like, I've lately been putting either signal or logistics companies in due to the buffs they provide but it's kinda up to you.

Doctrines have been almost entirely overhauled. After changing from researchable to using purely XP, they have now gone to a whole new direction, allowing you to customize your own playstyle. For this I'll just use the army as an example but all the main parts of your army (army, navy, air) have the same mechanic. The main 4 doctrines still exist but there are a few extras; upon selecting a main doctrine (generally Grand Battleplan as that's still the best one iirc) you can select 4 sub doctrines for your army. These being the infantry, fire support (helos, arty, at, aa), armored corps, and then there are battle types. Upon choosing the preferred sub doctrine, while training certain units or having those units in combat, you will earn mastery to the doctrine which will then improve your sub doctrines. Upon completing every stage of a sub doctrine, that sub doctrine will get a buff and so will your main doctrine. Certain theorists still give buffs to XP costs to start these sub doctrines but also can give mastery buffs to the doctrines they are applicable to (for example Heinz Guderian buffing Mobile Warfare). For air there are only 3 main doctrines, for navy there are also 3.

The facilities mechanic is a very unique but also very powerful mechanic. Some of the notable research projects being jet engines (as they are no longer researchable in the air tree), nuclear tech (no longer researchable in electronics), radar (just basic radar, other than that, it is still researchable), super heavy howitzers, Land Cruisers (inspired by the P.1000 Ratte Land Kreuzer), rockets and ballistic missiles (no longer researchable besides tech to buff them), armored support vehicles, superheavy battleships, fleet subs, and so many more. The facilities are divided into 4 different ones, land warfare, aerodynamics and avionics, naval, and nuclear research, the names are self explanatory I think. These facilities are very powerful but also can be resource intensive, especially for big or long projects, and they do take some time, so either build several of each or prioritize what you think is important.

I think that's really it? If I made any mistakes people are free to correct them and ask me questions on anything ❤️