Inside the ZIP file
There is currently no universal standard on how to distribute an avatar for VRChat. I have seen projects come in all sizes and shapes. What I consider the gold standard is a zip file that contains several folders that are containing things like texture files, PSD files, FBX/Blender files and usually in the root a readme.txt plus unity package file.
[need image with gold standard]
This is however not always the case.
It is possible that only a unitypackage is provided with no extra information or files. This is not really the ideal situation, especially the lack of resources to modify textures and mesh files can be disappointing. And without a readme.txt file you will need to look up the booth website every time you forgot what shader to use since a readme file will usually tell you what shader you need.
But before you get too disappointed, make sure to check the content of the unitypackage file. It is possible that additional files like blender/FBX and PSD files are included in there.
If they are included with the unitypackage file then you will be able to find these files in the project folder after you imported them to your Unity project. This can potentially bloat the size of your project folder, but in some workflows it makes sense to have the files there.