I know others will expand on this, but in the past there were two main “bases”: Debian and Enterprise Linux (EL). The main differences were their package managers and how the handled things in init.d and configuration like networking. This was due to how they made their modules iirc.

So a lot of distros forked off of these two bases rather than reinvent the wheel. Ubuntu is based off of Debian and CentOS based off of RHEL.

There’s probably more nuances but that should give you an idea.