Strategies for Composable Modular Configuration?

[Psionic K]

AFAIK, if we compare Emacs configuration to NixOS configuration, Elisp
lacks the composable module system.

A consequence is that users cannot easily delegate out the declaration of
configuration in whole or in part.  If I like someone's org mode defaults,
they cannot provide and I cannot consume that configuration except manually
because infrastructure for declaring its boundaries and how to compose it
with my own modifications doesn't exist.

What we would do with a composable module system is, for example, install
and configure a coherent set of minibuffer focused packages but without
getting in the way of the user providing extra configuration on top or
overriding attributes of the module either in a first-class or adhoc way.

1.  What prior art can I look at
2.  What implementation strategies appear viable
3.  Did DOOM and Spacemacs get some things right?

Thanks
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