Base Strike: In HOI this is the Carrier focused doctrine...technically, at game start NO ONE was carrier focused. The IJN was turning towards a carrier doctrine, the USA and RN were experimenting with integrating them into their carrier fleets, but no one was building true carrier fleets in 1936.
It is not just carrier focused (I think focused on a particular ship are wrong to describe the difference between the 3 doctrines). I would call it the flexible or balanced doctrine. In HOI4 it did not have the best convoy protection or best convoy raiding but it was not worst in either. Yes it do give more benefit to carriers then the other two doctrines but given the flexibility carriers and it's aircrafts posses I think it is fair. The doctrine is thus for the ones that needs a navy that can do everything in a good but not great way. US is a good example who need the ability to strike down Japanse convoys (and navy) as well as protecting its own convoys from German raiders.
Fleet in Being: In HOI this is the Battleship focused doctrine...every country in the game in 1936 was probably this at game start. The IJN was moving away from it due to the limitations of their shipyards. The USN was forced away from it by the destruction of PacFleet at Pearl Harbor. Germany realized they couldn't compete with the RN and switched to convoy raiding when they realized they couldn't afford Plan Z.
Again I think it is wrong to look at this doctrine just as Battleship focus. The whole point of this doctrine is to make it as hard as possible to sink your convoys. Thus this doctrine probably carry with it the most effective escort mission as well as the most effective patrol mission. It also most likely have the best ASW. The focus on Battleship also play into the defensive focus on the doctrine, for an offensive navy the Battleship slow speed and high cost may not make it a good choice but speed may not be as important for a battleship that escort convoys. It outranges all other ships but carriers and got the most ability to take damage, and damage dealt to the Battleship could mean less damage dealt to the convoys.
Just because Plan Z was supposed to be made of differen't types of surface vessels don't make it in HOI4 world a Fleet in Being navy because what is important here is how the navy would be used and Plan Z was a pure Trade Interdiction navy.
Trade Interdiction: In HOI this is the Submarine/light units focused doctrine. No one really was on this path in 1936, most that moved to it were forced onto by the realities of the war.
Again it is not the ships that make the doctrine but how they are supposed to be used. Nations that employ this doctrine are nations that will use most of its navy on convoy raiding missions as this doctrine are all about making the life of enemy convoys as difficult as possible no matter what ships you choice to use. Yes it have the most benefited submarines but submarines are about as pure offensive you can get as submarines have little defensive value. Yes I would say Germany had Trade Interdiction in 1936 because the whole idea was the need to destroy enemy convoys to win.
Looking at the icons for Base Strike, I don't see one that indicates the doctrine of coordinating multiple carrier air groups like Japan's First Mobile Striking Fleet or the later war American Fast Carrier Task Force. I was hoping Japan would get an early edge there since Japan's unique (at the start of the war) ability to combine six carrier air groups into a single striking force was a tremendous advantage.
Maybe picture 4 in the right column (the one which aircrafts bomb a ship). I could see Japan start with more of the doctrine researched then US. Given that Carriers aircrafts will also need air doctrines resarched to be effective, you may find what you are looking for in the air doctrines.
I don't think "attacking" or "defending" is particularly meaningful in the naval context. Both sides try to exert control over an area of sea, and that involves denying the enemy the ability to sail in that region. Eliminate the enemy's combat assets, strike their soft targets, protect your own. You can have situations where one side has the preponderance of the soft targets in theater under their banner, or a side has combat assets that are extremely difficult to bring to battle (IE: submarines) leading to a more 'defensive' character of the engagements (as the only options left are convoy defense / retaliatory strikes on soft targets), but the goal is still the same whether it's "your" waters or not.
Well saying that attacking or defending is not meaningful in naval context is about as correct as saying that attacking or defending is not meaningful in the land context. Yes the ultimate goal in the end is the same for any navy but the ultimate goal is the same for any army as well.
The difference between the 3 doctrines is how they use their resources. The Trade Interdiction doctrine focus on getting to enemy convoys and sink them (and any enemy warship that make itself a target). The Fleet in Being Doctine instead use its resources to make what Trade Interdiction tries to do as hard as possible. The Base Strike doctrine admit the need of both protection your convoys and sinking of enemy convoys and look to do both as well as looking for enemy warship to sink.
The ideal case that whole enemy fleet is gone is very far fetched, so the 3 doctrines offers you the choice of what you think are the most important role of your navy. Why I emphasis the convoys is that the only role a navy has is related to convoys (deny/sink enemy convoys and protect your convoys).
What I call offensive actions is everything directly releated to sinking convoys (including sinking enemy warships who are protecting convoys or hunting raiders) and defensive actions are directly tied to defending your convoys (including activly search for and sink raiders).
Yes they all try to reach the ultimate goal but it is what they focus on most that defines the doctrine.