Apple iOS has two core features when using a Password Manager to login. An app must have the "Autofill Password Entitlement" so the the autofill password feature in iOS appears on the top of the keyboard when on the login page, this allows you to access your password manger; without this you can use a password manager. The next core feature missing is the "Associated Domains Entitlement", this is the second part after autofill password implementation in the app. Associated domain set up on the apps login page will then allow a users saved credentials (username and password) to be saved with the website (domain) so it can be linked to that apps login page. without this you have to search through your long endless list of save passwords.

See apple documentation:

-- APPLE PASSWORD AUTOFILL --

(1) https://developer.apple.com/documentation/bundleresources/entitlements/com_apple_developer_authentication-services_autofill-credential-provider
(2) https://developer.apple.com/documentation/security/password_autofill/about_the_password_autofill_workflow

(3) 
https://developer.apple.com/documentation/security/password_autofill/
(4) https://developer.apple.com/documentation/automaticassessmentconfiguration/aeassessmentconfiguration/3566377-allowspasswordautofill
 

-- ASSOCIATED DOMAINS (For LOGIN PAGES) -- 


(1) https://developer.apple.com/documentation/bundleresources/entitlements/com_apple_developer_associated-domains
(2) https://developer.apple.com/documentation/xcode/configuring-an-associated-domain
(3) https://developer.apple.com/documentation/xcode/supporting-associated-domains