I don't know about the in-game stadium builds, but if only so that it's written down somewhere we can get to later, the externally-built stadiums I've made and seen have:
.obj file: this is the guts of it, and where all of the structures and surfaces inside and outside the stadium are defined, along with names for all of the textures (images and/or colors and/or patterns) displayed on those surfaces. This is output from Sketchup. It's just a text file, though I have only ever tried to edit it under duress.
.mtl file: this is also output by Sketchup at the same time as the .obj file, and it has pointers to all of the actual image files used in the .obj file, along with some lines saying how they should be displayed. So, this is the file that has to get tinkered with to make the animated crowd work, and it's also the file that will help diagnose what's wrong if something looks weird with a texture. It's a text file.
Textures folder: this is the last piece output by Sketchup, again at the same time as the others. It actually holds the image files pointed to by the .mtl file. So, in principle, everything pointed to by the .mtl file should be in here (other than the pointer to the animated crowd). If you have an unexpected texture on a surface, one of the main culprits is missing an image in this folder, whether because it isn't a png or something else happened.
.ootp3 file: this is made by OOTP at the time that a stadium is created. This holds all the camera angles and the grid. This is _not_ a text file, or at least shouldn't be treated as something you can freely edit. I have, however, successfully copied it from other parks and renamed it, which serves (if you're careful) to set the home plate and foul line positions as well as the camera angles very easily (the grid still needs to be made).
.prk file: I believe this is only made on demand within OOTP. It serves (if I remember right) to compile pointers to various files and also includes weather and the dimensions of the park. In theory, I believe one can play without any .prk files if one is willing to do a bunch of importing by hand, and depending on when the .prk file is made relative to the park being done it may not quite be right. But for importing external parks, it is definitely very simple if there is a .prk file.
Not sure if this helps here specifically, but as I said, at least it's now all in one place.
