How is the current adjusted from a power bank with adjustable current?
When you're regulating based on current you are actually converting that current to a voltage. This is done either through a shunt, a current transformer, or some hall effect sensor. You then end up regulating THAT voltage in the same way you would regulate your output voltage.
It really goes deeper than that depending on how you're regulating, but for voltage mode control that about sums it up.
EDIT: I should mention that I'm talking about SMPS, there are other ways of creating adjustable current sources that don't work like this.