Is unstable directly upstream stable version of packages exactly like Arch? Cos that is what I want.

Cos in Linux distro context, debian or nixos unstable repo just means it is not yet tested for the specific distro. But in fact the packages itself would be latest stable versions from upstreams.

I did read my reading but it is really blurry and hard to find information on how often the packages gets updated.

I also would like plain upstream versions of software without a distro doing any changes. And with NixOS and Nix, this is perfectly capable of fixing even the remote issues of instability that usually happens with Arch Linux.