This is a great question.
I work on Upwork as a freelancer as well as a client.
I will tell you what I personally do as a client, who has hired over 80 freelancers on Upwork:
I include all of the job requirements in the original job post. Either as text within the post itself, or as a link to a project description file on a webpage, or as an attachment.
I include EVERYTHING that a freelancer needs to know in order to finish the project.
I know that won't work for every project. But if it is possible for your project, then this is a great way to eliminate any confusion. This helps me to get good, qualified freelancers to bid on my project, and I am able to get the work done quickly.
When I do this, I don't spend time "interviewing" freelancers or desribing what needs to be done. I don't need to, because they already have the instructions and input files they need.