There isn't a single one, but off the top of my head, the following package managers have been used to meet the same need (building Linux distros) but use distinct and relatively new paradigms or techniques:

  - Guix, Nix
  - Distri
  - Luet

(I'm personally partial to the 'functional' school, which means Nix and Guix. :)

One might also count package managers that are designed for building large, multi-language distributions of software but have never been used to build a whole distro. In that case, you could also include

  - Spack
  - Denxi

and finally you might also include tools that build on top of the traditional stuff, like `rpm-ostree`. This is what you're dealing with on Fedora Silverblue, SteamOS, openSUSE MicroOS, etc.

Also distinct from these but not particularly new are source-based package management systems, like

  - source-based Linux distro package managers incl.
    - emerge
    - paludis
    - sorcery
  - various ports systems, incl.
    - pkgsrc
    - Ravenports
    - FreeBSD Ports
    - Dragonfly BSD's DPorts
    - MacPorts
    - Homebrew

but those aren't really in the mainstream of system-wide package management, at least from my Linux-centric perspective, so I don't usually mean them when I refer to 'traditional' or 'conventional' package managers without any other qualifiers.

Anyway yeah, plain old openSUSE and Fedora are great choices if you don't want a declarative system or immutable Linux, and that is in part because zypper and dnf are best-in-class for their paradigm.

wow, thank you! but are there linux distros that use any of these "from scratch" or as their native system for updating the kernel and reconfiguring grub, etc? or, to ask a different way, could these systems do that?

NixOS and GuixSD are distros built entirely around Nix and Guix, respectively.

In a word: yes! That I know of, Guix, Nix, and Luet are the only ones with whole, bootable Linux distros built on top of them so that the 'package manager' also does configuration management in some form. (IIRC, Luet is more traditional here, it manages config files only state fully and partially, using hooks and triggers.) Those distros are very worth checking out! The operating systems built on them are GuixSD, NixOS, and MoccacinoOS, respectively.

Distri is a system-wide package manager, but its distro is a PoC. It has a small package collection and no users, last I checked. But it has some cool innovations and it's very fast, so if you want to play with building a distro on a next-gen package manager, it seems great!

I think Denxi could be used to build an OS in a similar way as Nix, but I'm not aware of any such effort currently. The same is true of Spack.

Haiku and RedoxOS have some fancy Nix-like package management features as well, in terms of rollbacks and multiple versions of transitive dependencies playing nice together on the same system, and maybe in some other ways, too.

Nix probably?

Yep, but I'd also count some package managers which have learned from Nix but chosen a slightly different path by relaxing the requirement of 'purity', or leaning harder on containerization features.

Besides the ones I mentioned in my reply which is a sibling to yours, I think the package managers for Haiku and RedoxOS fall into this category (but I can't be sure! I've never used them, so my memory of how where work is likely to have gaps).