Please see: Develop Extensions — JupyterLab 4.3.0 documentation
During development, jupyter labextension develop will take care of most of the things.
Building a “prebuilt” distribution would move this to however the outer jupyterlab environment is installed e.g. pip or conda, usually as a .whl file.
Or, if this really isn’t meant to be treated as software: runtime configuration to find extra extensions after jupyter labextension build.