Python API, if I well understood the doc after a preliminary reading, is based on pyside2, which wraps Qt libraries and framework.
For the Qt GUI, there is an UI Designer tool on the Qt website.
There is also an IDE on Qt, including an extension for Visual Studio (but only for Express version),
Therefore any development that does not touch a RL native object can probably be done with those tools.
That may sound limiting but it depends on the type of plug-in and how much intelligence/processing is located "outside" of iClone assets.
Instead as soon as the code interacts with RL native objects (which means it is executed within iClone), then we have only an output console.
Regarding the integration with non-python libraries, there is the "shiboken" wrapper library that is used to connect iClone internal objects to the Python engine but probably also to connect non-Python code ro the Pythin engine.
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