Apple’s password manager, previously part of the Settings app, has gotten its own app in iOS 18. While most of the features within it remain the same as they were before, there are a few notable changes here and there.
iOS 18: 18 New Features Coming to Apple's New iPhone Software Update | WSJ
One of them is the ability to add a website to an existing password. This feature allowed you to link multiple websites to the same password, something that was not possible before.
In the Passwords app, tap All to view your passwords. Select one of the passwords you want to add websites to.
Tap 'Edit' in the top right corner and tap 'Website'.