Platform engineering is the discipline of designing and building toolchains and workflows that enable self-service capabilities for software engineering organizations in the cloud-native era.

Platform Engineering

One of the companies I worked at had this list of questions and I believe that they work great for platform products as well:
1. Give an example of a good platform product. 
2. Why do you believe it’s a good product? 
3. How would you improve it?

I'd ask questions around adoption. How do you measure it, what challenges to adoption have you experienced or seen, how have or would you address those challenges?

To me this is the number one reason to have a PO for a platform - people building platforms often find it difficult to focus on the things that will lead to users adopting and succeeding with their platform, they focus more on what to build and how to build it. A PO should be constantly asking "why", and keeping a strong focus on what will make the platform compelling for its users.

One of the questions I asked was what “platform” even meant to them. Internal platforms are relatively new and not everyone has clarity around the intention behind building one