Are they essentially all the same. I know there are hotwire and flap types but, are the resistances in the hotwire type the same.

istr yes

er no... (unfortunatley)

all hot wire work the same, but each type/model has a different 'function' ie for a fixed airflow, different parts will put out a different resistance / output voltage - that said there are some very similar - Merc C class diesel is very similar to a VAG 1.9tdi 90 one, but a little higher output so can be used in it's place and will drive a little additional fuelling