Well for start there is some a > b > c > d lines but also for example in the heavy cruiser's the year 32 and 36 are special. Moust of the others follow the basic pathers of higher the beter.
Now for carriers there are the following types of carriers basic/conversion, Fleet carriers(more planes ie. bether offence starts), Armored deck carriers(bether defence ledd planes)
Now for subs they come in 4 main variations and x2(x3 for japan) sub versions.
First the 2 basic sub types SR and MR these are the basic sub types.
The SR is a short range cheap sub thats fast and cheap to build. usefull for italy( subs for thr med) and germany( CR near the British isles and west of spain). Operational range 1.8k+.
The MR is a more expencive but has nearly 2x the range of SR subs. Operational range 2.5k+.
The LR subs come in 2 main variants basic LR sub operational range starting from >5k+ and the extended range subs the extended range subs have LONG operational range like >11k+
Second all subs can be fitted whit 1 of 2(3 for japan) variants.(SSE,SS-L,SS-H)
1st(SSE) is the basic sub variant if not sure what to use. Use this one.
2nd(SS-L)is the large sub this sub uses more supplies has hevyer attack is easyer to spot and more expencive.
3rd(SS-H) for japan only Heavy sub this is basicaly sub on steroids the reason being the oxygen powerd torpedoes. Rest pof the country's used air.
Destroyers
These cover lot of different ship types in 1. regular destroyers, heavy destroyers, torpedo boats, figates and so fort.
In addition all of them can be customiced for different aspect of naval combat.
Sub warfere(ASW), surface fleet battle(fire controll, toropedoes, 610mm torpedoes(japan only)), air defence, scouting(scout).
Cruisers and Capitals. (CaD, CrD)
On these what moust people miss is that you can customice these as well. But due to limits of the current version(1.08) you need to be bit triky.
To customice them, you must build naval brigates that you add to the ready ship they take same time to build as the ships.
For example there are fast, large, armored, and heavy main battery for the capitals. Battleship and Battlecruiser.
But like much of C.O.R.E there are still more, reading the C.O.R.E manual helps a bit also for meny best teacher is reading the ingame text/values.
Hope that answers some questions.