You can “call” it the informal version but they’re quite different. だ implies you believe something is true, so you can never use か　to ask a question with だ, since it makes no sense to be asking what you just stated you believed was true. です works with everything. When using だ with な-adjectives you have to drop the な.

その車の社内はきれいな。－＞　きれいだ。

In addition to that, 「だ」is integral to a lot of grammar formations that cannot be interchanged with 「です」. So, marking them as “formal” and “informal” is really not the right way to go about learning them. In the beginning it’s fine to label them as such, but once you start to get further you’ll start to realize that they really are, as Crumb said, completely different.