Similar to frameworks and libraries, there is a large variety of platforms with very different use cases. This section is about identifying the most used platforms.
There are platforms on which developers can run their applications relatively freely, regardless of which framework or which language used - for example, Linux, AWS or Kubernetes / Docker. But there are also platforms for which a software has been specifically developed, so that the platform is almost part of the application. These include Apple Watch or SAP.
Two more highlights from the comments: On the one hand, the freelancer platform Upwork - this raises the question of whether it is considered as a source of work or as a way to outsource. On the other hand Solaris, which was perhaps mentioned for nostalgic reasons.