Hmm so you don't use rpm-ostree? Or ostree at all? Sorry I'm just having trouble finding some of the technical implementation details, seeing a lot of details on the UX though.
Fedora still uses rpm-ostree, when you do an update it's pulling from an ostree remote served from a server. bootc replaces that with just an OCI registry. We ship the `rpm-ostree` binary on the systems still. It's still used for things like adding kernel boot arguments.
Here's their diagram: https://bootc-dev.github.io/bootc/filesystem-storage.html
Generally speaking new users can skip the rpm-ostree parts and just start with bootc. I am not an expert in this, there's a rust library in there somewhere. Hopefully someone can help fill in the blanks.