There are now two Apple ID one is with gmail address , the other is for iCloud Apple ID address the passwords are kept same by you ( in fact the Apple ID password should never be repeated with different accounts , they should always be kept different ).
When you log in www.icloud.com using iCloud mail address the suggestions will be shown for previous account , as they were added in address book , and Apple ID address is also saved in safari preferences ( see the passwords section ) , enter admin password , you can click on the mail addresses , the two mail addresses with password will be physically shown .
its a mail suggestion that can be eliminated Remove obsolete email addresses in Mail on Mac - Apple Support
When I go to manage the alias, it says I have used 0/3 alias and it seems the iCloud address is a primary address.
Icloud address is a primary address as the Apple ID say example1@icloud.com is signed in Mac system preferences and it will be on the top list , there are zero alias that you have not created .