The ".sde" file is simply a ESRI file format that contains the connection information required to connect to an ESRI enterprise geodatabase.
Using ArcMap or ArcCatalog, you can right-click on the ".sde" file and choose "Connection Properties" to see all the database connection information. This includes the database host machine, the database name and the username/password to connect as.
This information should be enough to be able to connect using standard database tools such as your Oracle 12c client.
Be very cautious about using a non-ESRI application when connecting to an ESRI enterprise geodatabase, though. Especially if the data is either versioned or has archiving enabled. What you see at the database level is quite different to what you see at the ESRI geodatabase (ArcGIS) level. Deleted/updated records may still be visible, etc.
Be even more cautious about editing the data through non-ESRI applications. You can potentially corrupt the database by introducing inconsistencies. If you really need to edit data by some means other than ArcGIS, refer to: http://desktop.arcgis.com/en/arcmap/10.3/manage-data/using-sql-with-gdbs/overview-edit-versioned-data-with-sql.htm and http://desktop.arcgis.com/en/arcmap/10.3/manage-data/using-sql-with-gdbs/edit-versioned-data-using-sql-oracle.htm etc.