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.

$ ps -ef | grep lreg
oracle     3536 105780  0 17:21 pts/5    00:00:00 grep lreg
oracle    95761      1  0 Oct26 ?        00:01:14 ora_lreg_ORCL      -- >>>>> lreg for oracle db 
oracle   116911      1  0 Aug23 ?        00:04:24 asm_lreg_+ASM      --->>>>>> lreg for ASM instance 

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.