Next, those assets are converted into files which can be imported into Unity to assemble them together. Nearly every 3d asset you download from here will be in this format, as a .unitypackage file, and with that file there may be texture .png and .psd files in the case you want to edit the textures yourself.

For assembling an avatar ready for VRChat, refer to their guide. You can usually skip step 5 and you'll want to import the shaders the avatar uses before step 3. The shaders for western models are usually Poiyomi (Pro) which can be found here for example, and the shader for Booth (anime-style) avatars is usually liltoon which can be downloaded for free. I recommend adding additional packages like Modular Avatar—use their VRChat Creator Companion steps, or try out Alcom, but this post is primarily for using VCC—and optionally VRCFury to start out.

If you have any other questions or need further guidance feel free to ask. I can go into more depth for setting up your avatar, for example.