The is a video-guide on how to login to Coracle on nsec.app homepage, you should enter your bunker URL or your name@nsec.app
Don't share bunker URLs publicly - you have to put them where they belong, i.e. into Coracle :) Although I agree this bunker thing is confusing.
Nsec.app stores your keys in your browser - there is no 'admin key', all keys are separate. The password you specify is used for e2ee sync of your key between devices - so you can login to nsec.app with name and password and get your keys synched there.