Did we have to know they were human or anything about them in order to detect and understand (or try to) the design?
Well, we certainly didn't come to the conclusion that humans made them based on any aspect of the items themselves. Moreover, restricting the possible designers to only humans meant that we could easily come to conclusions about their manufacture and use.
In other words, it was due to the
a priori conclusion that the designer was human that we were able to determine the purpose and function of their design. So yes, knowing that they were human was
crucial to the analysis and detection of their design.
We didn't know who designed or built it but we know it was designed and built.
No, we know that humans designed and built it. That knowledge does not come from any aspect of Stonehenge, however.
IOW we know Stonehenge wasn't the product of nature acting alone.
Right, because we know humans were there to build it.
In regards to the evolution of life, humans weren't around till the end. With no other designer avaliable, we must conclude that no design occured - that the only remaining possibility, no matter how improbable you may find it, is the truth; all other conjectures are impossible.
It does not mean that aliens didn't design and build Stonehenge.
What aliens?