Thank you for the feedback on the terminology - I agree there is a lot packed into that pricing. To answer these quickly,
“Apps” represent your application that you’d like to let your user extend. You’d want an App in XTP for each of your environments, e.g. “cart-manager (prod)” and “cart-manager (dev)”
“Extension Points” are the places within “Apps” where plugin code runs, and can be well specified with a schema. This is the interface between the host app and the guest plugin. It contains the definitions of the plugins functions that the host can call, the host apps functions the plugin can call, and the types that go across that boundary.
Yes, lots of XTP is open source - mainly the Schema definition as well as all of the code generators that make it easy for plugin developers to build against the Extension Point interfaces. This short example exercise will give you a good idea about all that:
https://github.com/dylibso/xtp-integration-exercise
As far as XTP vs. Extism, XTP provides both a DX lift on top of Extism via binding code generation based on our XTP Schema - as well as a production-ready platform to intake, host & deliver plugins.
We also have a very capable testing framework and simulation feature that is partially open source and runs online in XTP platform to guarantee plugin behavior and performance before you run it inside your own application.