Postgres only stores the metadata - array names, dimensions, attributes, residency and, in EE, user accounts, etc. Postgres is just the "catalog", and we aspire to phase it out.
The SciDB config file (usually /opt/scidb/16.9/etc/config.ini) specifies a "base-path" AKA the "data directory". Thats where the data is stored. You can also use the "data-dir-prefix" directories to split that storage among multiple drives on each node.