How can I save my appleID and password in my “passwords setting”

They already should be there unless someone else's Apple ID is there. If you change your Apple ID, then your current Apple ID will be changed to the one replacing it.

See Change your Apple ID https://support.apple.com/en-us/HT202667. 

And, here is What to do after you change your Apple ID or 

Password https://support.apple.com/en-us/HT204071  (supplementary to what the fist link says)

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If you have changed your Apple ID and the change did not take on your phone, try using just:

What to do after you change your Apple ID or  Password https://support.apple.com/en-us/HT204071

At the top right, there is a + sign. Tap that and fill out the Add Password form.

Go to Settings/Passwords & Accounts/Website & App Passwords and tap that. After entering your code, you should see a list of lignin data. At the top right of the list is the + sign.

idmsa.apple.com and iforgot.apple.com