The composefs project is a generic utility to create and mount filesystem trees; it knows nothing about OCI or containers.
The containers/storage project is a Go library for storing and mounting OCI/Docker containers; it can optionally use composefs.
At the current time, the composefs backend is not enabled by default. Enabling it requires at least these flags:
[storage.options]
pull_options = {convert_images = "true", enable_partial_images = "true", use_hard_links = "false", ostree_repos=""}
[storage.options.overlay]
use_composefs = "true"
Beware that because there is no dynamic merging for the storage.conf file, you must carefully e.g. create a new version in either /etc/containers or overwrite the system version, but merge options which are set by default.