I found the following Database Architecture Diagram for Oracle Database 12c on Oracle’s website. It lists all the processes and the relationship between processes and other database components.

http://www.oracle.com/webfolder/technetwork/tutorials/obe/db/12c/r1/poster/OUTPUT_poster/pdf/Database%20Architecture.pdf

And below is a YouTube link which gives database architecture overview.
Oracle Database 12c Architecture Overview