I've been looking into these for quite some time and haven't gotten very far.
So i thought i'd share my observations.
In this post i'll be talking specifically about files in the data/plcraft
First things first, there is two formats in here.
SMM, and DAT.
There are 3 variants of almost every file.
HA, WM and BASE.
HA means Hangar
WM means Wingmate
BASE is the player craft when in flight.
The HA and WM variants have everything stored in a DAT file.
At the bottom of the DAT files is what i assume is names for items inside.
For example, xwing_ha.dat
The WM file is the same, just replace hangar with wm.
Now this is where the SMM comes in.
The BASE DAT file contains everything but the TPI and TPD.
Instead, those two things are stored in a SMM file.(With the magic of SMMJ)
This kinda makes me think TPI and TPD are the model data itself, while the rest are textures.
If anyone wishes to take a look, here are all the Xwing files https://cdn.discordapp.com/attachments/419711036837330956/586167566100070400/Rogue2-Xwing.zip