You can pull DS games apart into their files (think files on a CD but in this case a cartridge, new for the DS it was too -- the GBA did not feature it outside of homebrew) with a thousand different tools. (crystaltile2, ndstool, ndsts, ndshv, dzlazy/dsbuff, tinke, nitroexplorer2...). Doing so will show you nice directories, file names, extensions and sizes, and opening the files up in a hex editor will show you what is inside in raw hex and some text conversion which also tells many people a lot. This is usually step 1 in playing DS hacker.
The main 3d format for the DS is NSBMD. It can have textures in (as well as having material colours and some fun on the vertexes) but will usually be flanked by NSBTX files.
Animations then handled by NSBCA files.
There are a couple of extras that are rarely seen that do things like animations of textures (
http://problemkaputt.de/gbatek.htm#dsfiles3dvideomostlyunknown https://raw.githubusercontent.com/scurest/nsbmd_docs/master/nsbmd_docs.txt ).
Sometimes files will have different extensions, sometimes they will be buried in archive files (see things like NARC and CARC, though more than just 3d stuff can be stored in those, and games are certainly not limited to it), sometimes textures will be scattered around the ROM's code and not in nice easy to edit files.
http://kiwi.ds.googlepages.com/nsbmd.html is an older version of what nsbmd entails. Similarly vintage is a tool known as nsbmdtool
https://gbatemp.net/download/nsbmd-tool.28230/ . It fails a lot but might get you something.
http://www.romhacking.net/documents/469/ has a bit more.
There are some converter attempts but nothing like "install this plugin for blender, press import, enjoy" that you can reliably use. You can try
https://www.vg-resource.com/thread-30547.html if you want.
https://github.com/scurest/apicula also does things for many people. MKDS Course Modifier had some limited options.
Textures most people will look at tinke to help edit. Some tile editors will do some of the various texture formats but some have some aspects that not even the otherwise more extensible of tile editors will handle (usually multiple bits for alpha).
There are also completely custom model formats, usually for older games (the NSBMD format did not appear right from the start, indeed the Metroid Prime Hunters demo features something that clearly influenced nsbmd but was not) but some devs later on still did their own thing. In that case you get to have fun with the DS 3d hardware as most formats will follow along fairly closely with it (editing 3d models on the fly is resource intensive so tends not to be done).
http://problemkaputt.de/gbatek.htm#ds3dvideo
For creating them then people either manually do it (there are few enough polygons that you could reasonably step through them detailing all the vertex points, I usually step through things to edit them) or there was a leaked version of the SDK (and possible some more) so you get to find DS vintage versions of 3d modelling programs and possibly risk what it you get playing with SDKs.
Various emulators like no$gba (maybe the debug version) and possibly desmume, not sure what goes for melonds right now, will have 3d viewers of some form in them. If you are just grabbing coordinates to transfer to paper for your purposes then that might well do something too, and even if not you might even be able to pull it directly from the emulator's memory and decode it that way. If you are doing emulator methods do bear in mind that you can use cheats to get to far along in the game and unlock what you want, and many times much has been done by swapping files around -- you don't need to know a thing about how a format works to rename a file and overwrite another if there are all nicely named files sitting around (plenty of new to it all hackers have made nice "play as NPC/bad guy/secondary character/boss"/every character looks like NPC/... type hacks doing just this), though those that do know about formats will tend to fare a bit better.