Prior to 12c, PMON process used to handle the instance registration with listner, From oracle 12c, a new background process called lreg takes care of this instance registration, Which reduces the load from pmon process.

Basically, LREG process does below things.

1. Registers instance information with the listener.
2. It is a critical background process for each database instance (if it is killed, Oracle goes down).
3. It does everything PMON used to do and report: service_update, service_register, service_died in the listener.log.