A lot depends on version and platform....for example
[oracle@db20 ~]$ sqlplus scott/tiger@db20pdb1
SQL*Plus: Release 20.0.0.0.0 - Production on Tue Aug 4 15:16:43 2020
Version 20.3.0.0.0
Copyright (c) 1982, 2020, Oracle. All rights reserved.
Connected to:
Oracle Database 20c EE High Perf Release 20.0.0.0.0 - Production
Version 20.3.0.0.0
SQL>
[oracle@db20 ~]$ ps -ef | grep sqlplus
oracle 3628 3128 1 15:16 pts/1 00:00:00 sqlplus
oracle 3640 3059 0 15:16 pts/0 00:00:00 grep --color=auto sqlplus
[oracle@db20 ~]$ ps auxw | grep sqlplus
oracle 3628 0.4 0.4 116572 24116 pts/1 Sl+ 15:16 0:00 sqlplus
oracle 3680 0.0 0.0 112660 2204 pts/0 S+ 15:17 0:00 grep --color=auto sqlplus
but of course, all of that is really just covering off the basics, ie, stopping someone from idly seeing it in ps. Its going to be saved in your history, and some process attaching will probably see it as well. Its a convenience not a cast iron lock. If you want to do it properly, you use a wallet.