I’ve no idea how it works. I never used it until recently, preferring to invent and remember passwords in my head. I don’t really like password managers, for various reasons.
But as a concession to lockdown, with a motivation to register lots and lots of new accounts with lots of new websites, from supermarkets to GP surgeries to breweries , I gave in and started using this one. It generates a long complex random password, which is saved (encrypted) in iCloud keychain and available for use with all Apple devices that are signed in to same iCloud account.
I have encountered one or two amateurish websites that are incompatible as they set daft ‘password rules’ that, for example, prohibit some of Apple’s random characters. But FB is the only one that seems to be just plain broken.