How would you define a Platform Product Manager? And what are their main responsibilities?

As a Product Manager, you are responsible for:

1. Defining product vision/strategy/roadmap based on deep understand of customers, business and competitive landscape

2. Working with various cross-functional stakeholders and partners to execute on your roadmap and ship product/features

3. Establishing processes to gather customer feedback to continuously evolve the product

Platform Product Manager role is very different except the product is a platform and hence your customers are the ones using the platform. Exact customer persona will depend on the specifics of the platform. Success is typically measured by adoption of your platform, customer satisfaction and the business impact your platform has. See this question for more details on metrics: https://sharebird.com/h/product-management/q/what-metrics-do-you-focus-on-as-platform-product-manager 

Platform product managers are responsible for building a set of services and capabilities that are used by customers (internal or external) to build products or solutions. Operating systems like iOS and Android, Mobile app stores like Google Play, and Cloud platforms like AWS, Azure, and GCP are some examples of platforms that typically platform product managers will be responsible for.

Platform PMs essentially build foundational layers or capabilities that help internal teams or external companies leverage these components to build new tools.

Platform product managers have similar responsibilities as product managers in terms of defining, prioritizing, and launching capabilities based on customer insights. However, since platform PMs are building core capabilities that are consumed by others, they have additional responsibilities around creating an ecosystem by not only evangelizing and educating stakeholders on the platform capabilities but also making sure the platform is extensible, scalable, and performant.