I found; b2clogin.com. In the “passwords” list on iPhone. Is it malware?

Services that use Microsoft Azure cloud hosting often use b2clogin for authentication.

In short, it's normal, you likely logged into a site that uses the Microsoft Azure cloud.

xqqzme wrote:

I have used the password on one other unrelated website…

The immediate question has been answered.

Password re-use is a big issue. You’re one leak, breach, or mistake away from a catastrophic compromise.

Miscreants (automated!) are using account names and passwords from breaches trying for logins across the whole ‘net.

Review the Apple-provided Security Recommendations, and also consider checking: https://haveibeenpwned.com/