system
Given the following post, respond as if you're commenting on Reddit:
human
[WH40K] What happens if a mechanicus explorator fleet finds an STC that makes something very valuable, like a Baneblade, but it absolutely requires Xenos tech?

So, the STC is genuine and complete.  There is no denying it is from the Omnissiah.  But it also requires xenos tech like wraithbone or Necron crystals.  What happens?  It might not be a replicator, it might be a huge tank.  It's just something very valuable.
gpt
STC's don't really work like that. There are varying reports on how they actually worked, but the general idea is that an STC is an AI-driven blueprint generator that is designed to analyze what materials you *DO* have to tell you how to make anything you could feasibly need. Partial STC's may only remember how to make one or two things, but they still are capable of generating resource-specific blueprints. 

The idea of an STC was that you send one out with a group of colonists to some random planet. The STC looks at the planet and decides "ok here's how you use what you have to make a thriving colony". No STC would really require xenos tech for that function.