How to understand a complex system?

An architect often faces something unclear that he/she needs to figure out.
Where do you even start?

I look at such a system or process as a black box.
I ask what the Inputs and Outputs of this box are. This gives a quick and simple understanding.
While discussing the inputs and outputs, the clear boundaries of the black box become visible.

Let’s take Workday as an example. Its landing page says: “The enterprise AI platform for people, money, and agents.” – absolutely nothing is clear.
But if an expert tells us:
Its Inputs
– Employee data
– Job applications
– Time-tracking records
– Payroll information
– Performance reviews

Its Outputs
– Reports
– Executed payments
– Employee profiles
– Compliance documentation
– Analytical dashboards

Then it becomes clear what kinds of problems such a complex system solves.
Some time ago, I built a service for automatic analysis of new products.

But this method also works great for establishing common ground when discussing a product with an expert.

What methods do you use to define something unclear?