Thank you for your answer @fricklerhandwerk!
There is some additional info in discourse about it: https://discourse.nixos.org/t/pre-rfc-implement-dependency-retrieval-primitive/43418 It looks as inputDrvs is not accessable from within the NIX-DSL. The Pre-RFC wants to change that.
What I wonder, when having a look at inputDrvs is, that there is no distinction between input-derivations that are needed to be in the store when one wants to execute a program in the output of the target-derivation and those which are needed just to build the target-derivation (without executing anything). That probably means when one copies a derivation from a substituter, Nix makes sure that even e.g. compilers needed for building it are in the Nix store.