What can I do when there is nothing to do at work?

I'm a contract software developer and would consider myself mid-senior level in my skillset. I've been working for my current client for about 3 months. Every assignment they give to me I get done as soon as possible to the point that I sometimes don't have much to work on some days. Lots of down time. Over the last few weeks things have really slowed down. Every other day I reach out to my two project managers and the general system architect of the company to see if I can assist them with anything. 90% of the time they have nothing for me.

What I found odd was my department manager, who is essentially the boss of everyone in IT, sent out a mass email asking if they know anyone that can fill a role that requires about 5 years more experience than I have, but involves work very similar to what I was hired on for. I even sent my department manager a text message saying I would be more than happy to take on some of the work this new role is suppose to do once it is filled. In addition I told them I don't have much to work on right now. The department manager never responded to my text, but I could see they read it cause it was iMessage.

We are all working remotely cause of pandemic. There are lots of different internal tools that are used and if I would tinker with them would be essentially guessing to figure out exactly how they work and how I can improve upon them. I would need a businesses analyst to sit down with me to explain everything.

What can I do in this predicament?