What is b2clogin and do I need it on my iPhone?

It appears to be Microsoft related, so you may want to contact their support. You must have tried or logged into an app or other MS related site for their login to appear. You can also delete it - if it is needed, it will re-appear.

Well, I googled it and cannot get any further since I was asked to sign into my Microsoft account. So you need

to see if you have one.