You’ve stumbled into the “UXP” forums. UXP is a modern JavaScript extensibility platform.
Unfortunately, Illustrator has not yet adapted the UXP platform at the time of this writing (nor do they have plans for it in 2024 that I’m aware of). To develop a plugin for Illustrator (at the time of this writing) you’ll have to use CEP for a JavaScript environment… or alternately try the C++ SDKs.
This Getting Started Guide uses Photoshop examples but also applies to Illustrator: GitHub - Adobe-CEP/Getting-Started-guides: Getting Started guides and samples for CEP extensions
The full CEP documentation is here: CEP-Resources/CEP_11.x/Documentation/CEP 11.1 HTML Extension Cookbook.md at master · Adobe-CEP/CEP-Resources · GitHub
If you’re interested in the C++ SDK for Illustrator, download the documentation here: https://console.adobe.io/downloads/ai … actually my link seems broken 
In the Console you need to browse to “Downloads” and search for Illustrator:
