Otoy develops OctaneRender Standalone and SDK.
Some selected external programmers are developing the majority of external plugins. The plugin is a bridge between the host app and the current SDK. The plugin without the SDK is useless, but adding an external GPU renderer to a 3d app that is not design to work in real time on GPUs is not trivial. So, at the end, everyone should be payed for his effort, Otoy for developing the hart of OctaneRender and the external developers for the refined integration inside host apps.
Last but not least, don't underestimate the power of Standalone. It need far fewer resources compared with any host app with plugin. With v3, all plugins are able to export the entire scene in ocs or orbx format to Standalone. If your host app cannot have a specific Octane feature for whatever reason, you can export to standalone and have access to it. It's also possible to collect assets from different artists created with different host apps. And finally, working on the scene structure with nodes is super cool