This is an imperfect analogy, but think of updating between Windows 10 and 11 versus installing updates on windows 10 or win 11.
I have no experience with Fedora, but AFAIK at least in Ubuntu/Debian land, updates are installed from OS version specific package repositories. When the version of the OS is no longer supported, those repositories might not receive updates anymore.
EDIT: this is the main reason I have a rolling release distro on most of my personal machines. The package repos have the newest packages without having to update my OS major version every now and then.
To OP: this is a much clearer & better explanation for what I was trying to say.