Thank you for quick clarification.

Is it correct that no new Furniture.jar will be created in the \lib\ directory, but the imported file is saved in C:\users\xyz\Sweet Home 3D\furniture?

Imported libraries are saved in C:\Users\xyz\AppData\Roaming\eTeks\Sweet Home 3D\ furniture

FYI - On my system (Win 11) it is not under \Appdata\..., but in C:\users\xyz\eTeks\Sweet Home 3D\furniture\