Giving domain name

how can we give different domain names to different DocFields in a single DocType?

domain names? You mean modules?

we can restrict a DocType to a particular domain.
so can we do that for fields as well?

I don’t think you can and you need to.
In this maturity level of this product, if there is no such design until now, that may mean your design lacks proper basing. Can you share more to discuss further?