Hello
When security is breached
Safari shows an alert as shown below.
For details:
Section: Password monitoring
Safari securely monitors your saved passwords, automatically keeping an eye out for passwords that may have been involved in a data breach. To do this, Safari uses strong cryptographic techniques to regularly check derivations of your passwords against a list of breached passwords in a secure and private way that doesn’t reveal your password information — even to Apple. If Safari discovers a breach, it can help you upgrade to Sign in with Apple when available, or automatically generate a new secure password.
https://www.apple.com/ios/ios-14/features/
https://support.apple.com/guide/security/password-security-recommendations-sec7f0432063/web
See Also section at the bottom have links.
Best.