ostree is the library that rpm-ostree and bootc share. However bootc is moving over to composefs as a backend. This effectively makes it distro agnostic and there are communities forming: https://github.com/bootcrew

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.