ssl with postgresql 12 I tried following the docs for the same.
It says postgres.conf will have service.crt for ssl_cert_file, but it has /etc/ssl/certs/ssl-cert-snakeoil.pem 
and now I can't proceed with it.

Any insight or tutorial will be helpful.
Simple. Change the parameter ssl_cert_file to point to your server certificate and ssl_key_file to the private key that belongs to it.

If the private key requires a pass-phrase, you also have to set ssl_passphrase_command appropriately.

To validate client certificates, put the certification authority certificate in ssl_ca_file.