Jetbrains products inlay hints

i'm planning on developing a plugin that does an inlay hints just like java in intellij idea or c# in rider but for other language, and i want to see how jetbrains developers implemented the hints for java or any other language so i can get inspiration on how to develop it!

but i can't find the classes that implemented this feature, so what i want?

- I want to know where it is implemented (Specific Package path) so i can see how it works.

- I want you to back me up with any sources that could be useful in this kind of ideas or any information.

Note : The project is based on Intellij idea Plugin Template.

Thank you so much in advance! :D