Thank you! This is the page where I discovered that I can try rpm-ostree kargs --append=modprobe=zfs, which didn't work.
Still, as far as I understand it, ublue managed to overcome this, as they do offer a Fedora version with zfs included.
What am I missing?

I chose Fedora since I'm most familiar with RedHat from my professional activities.

This is where my knowledge of Linux falls short.

is the package you installed for ZFS a DKMS kernel module, or a binary one?

In both Fedora and Silverblue I find the same, ZFS-related files, under /usr/sbin/

 Jan  1  1970 blkzone
lrwxrwxrwx. 43 root root      11 Feb 28 13:59 cache_metadata_size -> pdata_tools
-rwxr-xr-x.  3 root root   24176 Jan  1  1970 fatresize
-rwxr-xr-x.  3 root root     756 Jan  1  1970 fsck.zfs
-rwxr-xr-x.  3 root root   15960 Jan  1  1970 fsfreeze
lrwxrwxrwx. 91 root root       3 Feb 28 13:59 lvresize -> lvm
-rwxr-xr-x.  3 root root   24104 Jan  1  1970 mount.zfs
lrwxrwxrwx.  3 root root      22 Feb 28 13:59 named-checkzone -> ../bin/named-checkzone
lrwxrwxrwx.  3 root root      24 Feb 28 13:59 named-compilezone -> ../bin/named-compilezone
-rwxr-xr-x.  3 root root   32568 Jan  1  1970 nilfs-resize
-rwxr-xr-x.  3 root root   69672 Jan  1  1970 ntfsresize
lrwxrwxrwx. 91 root root       3 Feb 28 13:59 pvresize -> lvm
-rwxr-xr-x.  3 root root   73784 Jan  1  1970 resize2fs
lrwxrwxrwx. 11 root root       9 Feb 28 13:59 resize.f2fs -> fsck.f2fs
-rwxr-xr-x.  3 root root   24408 Jan  1  1970 resizepart
lrwxrwxrwx. 43 root root      11 Feb 28 13:59 thin_metadata_size -> pdata_tools
-rwxr-xr-x.  3 root root     804 Jan  1  1970 xfs_freeze
-rwxr-xr-x.  3 root root  212440 Jan  1  1970 zdb
-rwxr-xr-x.  3 root root  107736 Jan  1  1970 zed
-rwxr-xr-x.  3 root root  149984 Jan  1  1970 zfs
-rwxr-xr-x.  3 root root   15920 Jan  1  1970 zfs_ids_to_path
-rwxr-xr-x.  3 root root   15920 Jan  1  1970 zgenhostid
-rwxr-xr-x.  3 root root   32472 Jan  1  1970 zhack
-rwxr-xr-x.  3 root root   65400 Jan  1  1970 zic
-rwxr-xr-x.  3 root root   36536 Jan  1  1970 zinject
-rwxr-xr-x.  3 root root  264288 Jan  1  1970 zpool
-rwxr-xr-x.  3 root root   57688 Jan  1  1970 zramctl
-rwxr-xr-x.  3 root root   40720 Jan  1  1970 zstream
lrwxrwxrwx.  3 root root       7 Mar  4 10:20 zstreamdump -> zstream
-rwxr-xr-x.  3 root root  162208 Jan  1  1970 ztest
-rwxr-xr-x.  3 root root   53384 Jan  1  1970 zvbid

Also, ztest does something

verifying concrete vdev 0, metaslab 11 of 12 ...
loading concrete vdev 0, metaslab 11 of 12 ...
verifying concrete vdev 0, metaslab 11 of 12 ...

I assume, therefor, that OpenZFS exists, however the kernel is not loading the related module.
Binary, library or something else - how can I tell?