I think you need to keep your posts organized and somewhat related. You are still getting the hang of things so maintaining many posts about the things you are learning may be counter productive for the time being.
I’m assuming you have successfully installed Asterisk. Did you install it following a similar path as the book (Asterisk The Definitive Guide 5th ed).
If you did, that setup is expecting to get pjsip details from a database and not the pjsip.conf file.
Anyway, install sngrep on CentOS 9 (sudo yum install sngrep -y) , run it by typing sngrep then attempt to register your endpoints and see if there’s any activity on sngrep