$ sudo apt install syncoid
E: Unable to locate package syncoid
It appears that syncoid is in "sanoid" package:
$ sudo apt install sanoid
$ syncoid
it shows parameters to be used.

The destination pool (in my case backup pool) to which i wanted to copy/mirror the data not exist, so i have created it:
$ ls -l /dev/disk/by-id
$ sudo zpool create -o ashift=12 -o feature@async_destroy=enabled -o feature@empty_bpobj=enabled -o feature@lz4_compress=enabled newpoolname /dev/disk/by-id/drive-name-returned-by-previous-command
Then ran the sync:
$ syncoid -r sourcepoolname destinationpoolname
Even it bothered with critical error (dest. pool exist, but no snapshots on it), it started syncing datasets apparently, except the content right in the pool but only sinced content in pool datasets.

Later there was a suggestion to use switch/parameter: --skip--parent which would skip files right in the pool and sync only datasets content.

Even later was suggested to use --no-stream which retains only most recent syncoid snapshot saving the disk space.

So the resulting sync. command that IMO may be suggested is:

syncoid -r --skip-parent --no-stream sourcepoolname destinationpoolname