the custom field library - when you go into create a field and you see the library tab - this is having a makeover soon
You can search by fields but it will give you the tasks with those values, currently there is no way of seeing which projects are using which fields (unless you have bundles)
In the custom field library you can add it to a project, click edit then delete.
I don’t believe this is possible - we recommend setting conventions when onboarding your team.