There is no one true way.
You know your configuration and check the implementation of some of the options you set.
There you will find that nano is basically installed always via environment.defaultPackages and that xterm gets installed by default when enabling X.
If you have the experimental cli enabled, you can use why-depends, which shows a nicer tree, which is helpful when something isn’t actually installed as a program in PATH, but instead just a transient dependency of the closure.
And even then it is a lot of guesswork based on names of store paths.