We are planning to Oracle database from 12c to 19c of ArcGIS Enterprise/SDE we use.
The Steps are so far:
1) export full db backup of current 12c
2) import full db backup to new 19c
what should be done regarding SDE database?
Be sure to import the SDE schema before any of the other data owner schema's.
Methods to move a geodatabase in Oracle—ArcMap | Documentation
Once you connect and confirm the import was successful, you could then upgrade the Enterprise Geodatabase portion. This could also be done before the export / import, as long as the 12c version is supported at the "updated" ArcGIS client level.
You should ensure that your enterprise geodatabase version, as well as the version of any connecting ArcGIS clients, is supported with Oracle 19c. Full support for Oracle 19c starts with ArcGIS 10.8. Oracle 19c is support with ArcGIS 10.7.x with a Support patch.
Oracle database requirements for ArcGIS
The following Technical Article describes the steps for importing an Oracle Data Pump export dump file by schema-- to import the SDE schema first to the new 19c database and check for any invalid SDE objects prior to running the full import.
How To: Perform schema restores of an Oracle enterprise geodatabase
Retrieving data ...