There are a few "grandfather"-distros out there, for example Debian and Arch. They've been around for a few decades now.
Then, they got kinds, because some people said "I don't like xy, I will do it better", but granddaddy disagreed, so they split apart.
That's what Ubuntu is to Debian for example, that's why Ubuntu is Debian-based. They are related to each other (e.g. the same package manager), but differ in some things (e.g. update cycle).
This cycle of forking continues, that's how Mint got there for example. Mint is based on Ubuntu, and Ubuntu is based on Debian.
But nowadays, the gap between distros gets smaller, with things like Distrobox, Nix, Flatpaks, and more.
I wouldn't mind working with a PC that has Mint on it instead of Fedora. Sure, there are reasons why I prefer one over the other, but in the end, they're all the same.
One example I can think of where the base matters, and not the package manager, is when adding an user to the sudo group. RedHat distros need another promt than Debian for example.
But other than that, the thing that defines a distro are the packages, they make a distro unique.