First pass at describing the Fedora update/upgrade/release process, based on a thread I started here.

This page may possibly be useful to you in understanding the details of this stuff:

https://docs.fedoraproject.org/en-US/quick-docs/repositories/

There are three things to keep in mind, broadly speaking: the Bodhi 'update' process, Koji tags, and the compose process.

When we do an F31 compose what we're basically doing is taking all the packages that have the tag 'f31' in Koji and producing some repositories and some 'deliverables' (i.e. images, basically) from them, all bundled up in this thing called a 'compose'.

If that compose is a nightly compose, and it's successful, the symlink you linked above will point to it. Also, its contents will be synced out to https://dl.fedoraproject.org/pub/fedora/linux/development/31/ … which is where the 'fedora' repository on an installed system goes looking for stuff.