I suspect many don’t write anything down and do this largely by intuition/experience but I want to tease out some ideas.

when it comes to describing and designing a system from a blank piece of paper, what are the parameters you think of?

I’m thinking napkin sketch level of software design.

So things like:
Number of users, are they concurrent users, what load dimensions there are (disk IO, network IO etc.), target platform (everything is a web app these days), how do you design/visualise the data model?

Any decisions or constraints that impact what and how you build a proof of concept / MVP? How do you document this? How do you test it against the finished software?