When iCloud Keychain is set up in iPhone Set up iCloud Keychain and iCloud Passwords on all your devices - Apple Support
The passwords are autofilled Automatically fill in strong passwords on iPhone - Apple Support
The Apple ID password can be found How to find saved passwords and passkeys on your iPhone - Apple Support
Another method is if you forgot Apple ID password , reset it .
Reset Apple ID password using find my iPhone .
On your iPhone device , click on home scroll to the next page > click on extras > you will find find my iPhone click on it , enter your Apple ID only , click on forgot Apple ID or password , click on next.
Reset options - choose how would you like to reset your password ( a )
Click on reset through mail and click on ok .
Then open your rescue mail with web browser ( server account ) , open mail > click on reset password > two fields will be shown .
( a ) new password …..
( b ) verify password ….
Enter the credentials , and click on reset password , the password is changed .
If you click on - answer security questions - click on the right arrow .
Date of birth , verify your date of birth to continue > fill up the credentials > click on next and same pop up window will appear .
New password ….
Verify password ….
Fill up the credentials and click on reset password , the password is changed .
See the article If you forgot your Apple ID password - Apple Support
