Beware using ZFS with external disks: https://github.com/openzfs/zfs/issues/3461

You can add devices as vdevs to a pool, though. You can't add disks to a vdev. (IIRC) It's no different to LVM in that aspect.

IIRC, the pool is effectively RAID0 over vdevs: if any single vdev dies - whole pool dies. Do i remember this correctly?

So if over the years I got, say, five disks of varying quality - I shouldn't add them to a pool as five individual vdevs :)

Correct. I lied when I said you can't add single disks, I meant you shouldn't add single disks. It's not like e.g. Synology where you can add a single disk and it will expand your underlying RAID5/6 arrays

> if any single vdev dies - whole pool dies. Do i remember this correctly?

Yes, that's why you don't have single-disk vdevs.