iCloud email password that is NOT my Apple ID

If the mail address that is example1@icloud.com is always an Apple ID yet not signed in Mac system preferences ( in fact you had a different Apple ID right now signed in say example2@gmail.com its password can be found if you have set up iCloud Keychain https://support.apple.com/en-in/HT204085

https://support.apple.com/guide/ipad/passwords-devices-ipada39a7fa0/14.0/ipados/14.0

from the keychain access of Mac https://support.apple.com/en-in/guide/keychain-access/kyca1085/mac

Open keychain access in the section iCloud use its spotlight search and type the mail address in it , you as an administrator can only find it ( it will prompt to enter admin password ) to find the password for mail address .

About switching address there are two ways change from appleid.apple.com see the article https://support.apple.com/en-us/HT202667

If iPad is also synced with Mac then you have to follow https://support.apple.com/en-in/HT204071

The second way is sign out existing Apple ID address from Mac system preferences and sign in with desired mail address .

The mail address could be created with some mail services like gmail , yahoo ....etc that is not signed as Apple ID , you can change the password on web server .

For gmail accounts see the article https://support.google.com/mail/answer/41078?co=GENIE.Platform%3DAndroid&hl=en