Is this what you are looking for? Custom UI Kits – Adobe XD Feedback : Feature Requests & Bugs
Please vote for this feature request, or look up similar ones on the site. If you don't find any, create a new feature request.
Partly. I guess what I'm saying here is that it would be great to be able to implement UI kits and then get access to those via the left menu. I picked up a site that had UI kits for Xd but I can't really see how to utilize that as its not possible to implement those. Are they meant to be Xd project files? In PS you have brushes and custom shapes integrated into the respective palettes so to speak, but in Xd I can't really see how UI kits are possible to implement?
Definitely upvote for the idea that Preran posted to you. The UI Kits in the products are simply XD files that are available from the menu.
Again, I'm not sure if I understand what your saying. "The UI Kits in the products are simply XD files that are available from the menu." But it's not. The UI kits for download are Xd projects - right? Can't see any of them integrated in the menu.