The closest equivalent to the nixos ISO builder and reproducibility related to it in the fedora ecosystem is osbuild / imagebuilder - https://www.osbuild.org/guides/introduction.html

Imagebuilder claims reproducibility, but as far as I know it mostly installed rpm packages as binaries, not from source, so it's not really proper reproducibility unless all the input packages are also reproducible.

If the descriptions of building packages from source, building distro images, and reproducibility in the linked thread didn't make sense to you, you're probably not really the target audience anyway.