Searching example to overlay two root filesystems at boot (rw(rw) - initramfs

[mabra]

I am trying to run a kvm box with zfs on root (which is generally
no problem) with a read-only root-pool and a second pool (read-write)
which have to be mounted using overlayfs in initramfs.
I have never done something like this (searched web+github), but
read this: "Read-only root with overlayfs"
( https://gsthnz.com/posts/readonly_root_with_overlayfs/ )

If someone could point out resources about this or knows
instructions, please share it!

And sure: It has nothing to do with blockdevices/clones.

Thanks!