I am quite definitely not a very artistic type so my advice may be of marginal use! I am a programmer and sysadmin by trade, though I suppose I did actually do very well at engineering drawing back in the Dark Ages.
To do pretty much anything with video games you need tools to work on textures and meshes. The no-budget approach to these is nearly always Gimp and Blender respectively, but people with a budget (or who already use the software) will use the likes of Photoshop and 3D-Max or some other equivalent. I don't use those so can't comment about them.
Those two tools will also need add-ons to convert between the relevant formats. Most video games use DDS files ("DirectDraw Surface") and most applications don't support it natively, helpfully. I forget offhand which plugin I use; I
think it is the nVidia one but I'm not certain. I have a feeling it's a bit older, but most games use the older formats anyway, typically DXT5 if there's transparency and DXT1 if there's not, and with mipmaps (i.e. decreasing ¼-size copies of the image) for files that are displayed in the 3D world but without them for 2D stuff show in e.g. menus. The textures often have at least three components: the base or diffuse map (i.e. what colour it is); the normalmap (how bumpy it is, to give the texture some, well, texture) and a specular map to control reflectivity and stuff.
Blender is something that takes a bit of getting used to and in over a decade that still hasn't really happened with me.
But I figured out enough of the basic so create and alter models. It can be used for static things such as architecture and movable things such as characters and what they're wearing. The exact implementations vary from game to game but typically your moveables will need an armature (skeleton) and your statics will need a collision box. Hopefully the game will be documented somewhere in sufficient detail (usually by the modders) for you to figure this out.
For BG3, you need two main tools: Norbyte's tools over on Github, which convert between the GR2 "Granny" files used by BG3 (this is a proprietary format popular with developers and unpopular with modders) and an intermediate one which can be read by Blender, in this case Collada (.dae). As well as converting the files it also deals with unpacking the archives they're stored in and decoding the "binary XML" configuration files it uses. Laughing Leader's tools are not essential but useful for finding problems and so on.
I think that explains the basics... very poorly, so I do apologise for that! And something you probably already know but which is always a good reminder, most of these files are not suitable for actually storing your work: DDS is lossy, and alongside MP3 is a sort of digital equivalent of cassette tape, so keep the originals in Gimp/Photoshop or at least PNG format. Similar with Blender: though Collada presumably isn't lossy, saving your stuff in Blender's own format makes life a lot easier. Oh, and backup, often!
