So majority of distros you see are typically based off of another, as you’re already aware.

We have Debian based distros such as Linux Mint, Ubuntu, Kali Linux, PopOS and so on, then we have Arch based distros such as Arch Linux, Manjaro, EndeavourOS and so on, and then we got Fedora based distros such Fedora, Nobara, RisiOS.

You get the idea.

Now if Debian were to move a package from unstable to their stable repository all these derivatives will also pick it up however, they can prioritize their own packages from their own repositories over the ones supplied by Debian.

If you’re using Linux Mint cat /etc/apt/sources.list and also check out the files in /etc/apt/sources.list.d and you’ll see the repositories your system pulls from. You can add repo’s there if you want but you have to be cautious that the packages in the repo don’t conflict with the ones already installed, that means ensuring the versions are also supported on your distro.

I’m sure others can give more detail on Arch and Fedora based systems as I am not personally well familiar with those distros and how they store their sources files.