PostgreSQL doesn't log
The logging collector tries to capture stderr and write it to a file.  But it looks like systemd wins the redirecting war, and sends the output to the journal.  You can check the systemd journal with:

journalctl -u postgresql


The systemd configuration is in usr/lib/systemd/system/postgresql.service. I've tried various settings for StandardError, but none seems to allow the regular Postgres logging to do its work.