Hi! So, what you just bought is a zip file containing an fbx file (the model) and a png file (the texture). What you need to do now is to import the fbx into unity and add the texture in the material tab, I'm sure you will find a tutorial for it online. It is packaged and sold like this so that you can use it in any program, not just unity.
That's not what i'm asking, sorry let me be more clear on my words. How do I import the FBX file into unity if the FBX file is NOT a unity file? it comes up for me as a separate file that goes directly to a non-supported software called Krita? how do I changed the location opener for the FBX file to be imported into unity and not Krita?
I'm guessing what you did was at some point you chose Krita as the app for opening FBX files. You can change that by rightclicking the file, choosing properties and then clicking "change" next to the "Opens with:". All in all that shouldn't matter if you're importing the file directly into unity by using its import feature/dragging and dropping. I recommend you look up a tutorial on YouTube for VRChat/Unity that shows how to put clothing (like shirts, jackets, etc.) onto a VRChat avatar when the clothes come as an FBX model, not a UnityPackage. Make sure the tutorial also explains how to properly link or assign the bones (armature) so the clothing moves correctly with the avatar.