I actually had a video on this, but it's a tiny bit cringe because of age and that was before I learned more about public speaking. Actually no, I redid those, audio is kinda echoey though, not sure why.
To attempt to be brief, a good example of a Doctrine came from Lubyak. Lubyak stated on their doctrine that they believed the most probably serious threat was an invasion by one or more nearby nations, which were economically or technologically more advanced, and they believed any possible wars would happen within Lubyak territory. Their solution was attempt to squeeze their resources and try to make the most of them, while also making use of their own territory and their superiority in numbers.
Think of it as the basics of what Sun Tzu taught. Know your strengths, know your weaknesses, know your enemies' strengths and weaknesses. Bolster your strengths, cover your weaknesses, dimish the strengths of your enemies, and exploit their weaknesses.
Now doctrine feeds into the "top and bottom end" of the strategic/tactical situation. At the top, you have divisions which are around 10,000 people and they used to be the smallest unit that could be deployed into a far away land. They would typically be two thirds one thing, and one third another, but sometimes it was three fourths and one fourth.
My video also goes into this, but again, too long.
The US has moved onto Brigades, but that's a lot harder to setup and is harder to explain, so I am going to pretend their Brigades are divisions, like they used to be.
The US currently has about half a million in the Army, with the bulk of that being logistics or people currently at home. They aren't the most helpful resource for nations with billions of people, and equivalent per capita military spending. You're going to see why in a second.
The US has armored brigades, Styker brigades, and light infantry or "Humvee" brigades. We can convert these into divisions and use them to understand how you or other NS nations might do things.
"Light Infantry" divisions, mostly revolve around infantry on food, which may or may not be using pickups, 4X trucks, humvees, Landrovers, or MRAPs. These units are cheap and typically are used by the US for patrols or to get into areas that are tricky to get into. If you aren't "nation-building" or trying to fight in places that heavier vehicles can't get into, you might have light infantry units be purely special forces, reserves, or national security units (India deals with a lot of terrorism and has military police divisions).
"Mechanized Infantry" units revolve around APCs, which is an armored box on treads or typically 8 wheels. In the past they had little armor and were used as "battle taxies" but lately they've had a lot more armor and have gotten a lot more mobile as well. The Stryker is a good example because the unmodified version can technically be stuffed into a medium-sized plane and flown half way around the world, but the vehicle can fairly easily be mounted with add-on armor that provides a good bit of protection. The Mech Inf division is typically the "bread and butter" of most armies, as it's able to do almost everything reasonably well.
"Armor" Divisions or units, revolve around tanks, typically MBTs for richer nations and medium tanks like the T-55 for poorer ones. These divisions might be 1/3rd tanks or 2/3rd tanks, but at least a quarter of the unit is Infantry Fighting Vehicles and logistics for those units, typically being whatever portion isn't tanks or logistics or support units.
My video goes into calvary, airborne, heavy armor/breakthrough, and artillery units as well. Again, too long, don't have time, you're busy, I'm busy. Just remember that what kinds of divisions you have and how many you have, is based on how mobile you need units to be, and how much you can afford. Some nations are top to bottom armor divisions as much as possible, because they see wars being fought over small spaces, and they are made of money. Armor divisions take FOREVER to get places, especially by boat. Most of the tanks in the US get around by railway, because they tear up roads. If tanks don't tear up the roads, the roads tear up the tanks.
Let's get down to squads real quick before you lose the will to live and/or enter a coma.
A squad/fireteam of infantry is sized based on however many people you can stuff into the vehicle they use to get around.
What I mean by this is, if the Humvee holds a drive and gunner, along with three others... so perhaps a fireteam might be three to five people. If that is the case, perhaps your squad is 6-10 people. If you have a vehicle and it holds less then seven people, including the commander in a turret, that vehicle is holding a fireteam. If a vehicle holds seven to 12 people, that's a squad.
So example, Styker holds nine people including the commander of the vehicle, so those units have nine men squads. Bradley and Puma IFVs carry six people, but the commander of the vehicle could be the Sergeant of the squad, and thus be the seventh man.
Quick rules for how a fireteam or squad is "layed out". A fireteam always has one LMG or GPMG, in the same caliber as the service rifle, if possible. If you can't get the same caliber, you use .308 or 7.62x51mm. This is how things have been done in most the world since 1945ish. If your fireteam has at least four people, one of them is is the fireteam leader, and one of them is carrying an underbarrel grenade launcher (If you can afford it). In the US army, GL and fireteam leader are same person. In the US Marines, they are separate people. Most of the world copies the Germans from WW2 and has everyone carry as much spare LMG ammo as they can, typically at least a 100 round belt or two 45 round magazines (Maybe 50 rounds if it's heavy .308 or 7.62x51mm)
Soviet squads or seven men squads are tricky. Soviets would only have one LMG and always have an RPG-7. Typically anti-tank weapons are only issued out if tanks or vehicles are a serious concern. Also, a lot of nations issue out disposable launchers and have the vehicle carry them in case someone needs them, such as the LAW, RPG-18, ect ect. I advise you always bring two LMGs if you can, unless you absolutely have to have a dedicated two guys with an RPG-7 or equiv in your squad.
That's the fundamentals for you. Let me know if I talked down to you or went way over your head. I know a lot of this might seem like too much nit-picking, but I think that little stuff like this can make your nation seem way more capable in RPs, way more interesting in writing, and can be quite easy to do if you get down to it.