I believe ZFS at least used to have an option to insert itself directly into a Linux source tree, in which case it would look just like a normal driver. I don't know if that still exists and I never tried it, but it was a thing. Note that you probably still need an initial ramdisk to get the userspace tools to actually bring a pool online if you're using it for root.

Edit: I'm having trouble finding it in the official documentation, but here's a page that describes how to do it on an old version: https://slackwiki.com/ZFS_root_(builtin) and here's what looks like a script to do that on the current tip of master: https://github.com/openzfs/zfs/blob/master/copy-builtin