You know some. You don't need to know every variable to create an estimate.

Critics of estimation often use a sneaky trick called the motte and bailey fallacy. They mix up two different ideas: one that's pretty reasonable and easy to defend, like "we can't know _exactly_ what's going to happen in the future," with another that's way out there, like "the future is a total mystery, so trying to learn anything about it is useless because it's all just random noise."

I think for that scenario, there would have to be experience estimating based on the third party involved.

We have third parties building portions of our systems and for some of them I can estimate reasonably well their actual net duration because I've had a lot of experience with them, and they've been fairly consistent.

But there are others where there's a wide range, so I think it boils down to whether you can spot a pattern or not.