If you can run the "pg_config" binary, it should be able to show you where SHAREDIR and LIBDIR are. Extension files are kept in $SHAREDIR/extensions and any library files that may be associated with them will be in $LIBDIR. Example for my custom build...
My old computer broke and I've completely forgotten where OpenSUSE stores its PostgreSQL extensions. I've been searching with DuckDuckGo but no luck. Is there a quick command to find out where to put them?