Reading the following threads has relevant information on the topic of extensions installing python packages for Slicer installed to a System rather than User location.
The application is fully portable, including all data and settings.
However, you need to install in a writable location if you want to allow your users to install extensions (as all the extensions and additional Python packages that they pull in must be all available to all users). It might be possible to remove this requirement (e.g., install additional modules and Python packages in the user’s folder), but it may not be easy to implement this and there has not been very strong demand for it (…
It’s not set up for that. Better to either pre-install all extensions in the read-only directory or give each user their own copy of Slicer to install extensions in.
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