it is everytime the same.
First Oracle release 19c for the Exadata Cloudmachine.
After the Exa Cloud Machine 19c will come for Onpremise customers with Exadata.
But the exact date knows only Oracle but this is the MOS note.
Release Schedule of Current Database Releases (Doc ID 742060.1)
Have fun :-)
Oracle database 19c is released for exadata on-premises on 13-Feb-2019. This time it is released for on-premises before cloud.
It is already available on exadata.
You can check mos note for more details.
Exadata Database Machine and Exadata Storage Server Supported Versions (Doc ID 888828.1)
Below patch is already available for download.
Patch 29181103: EXADATA COMPUTE NODE 18.104.22.168.0 OL7 BASE REPOSITORY ISO