The emails, which were deemed to have contained/referenced classified material, that were found on Hillary's server, were only given that designation AFTER they had been mailed to/from Hillary. In other words, at the time that they first uploaded to her server, they were not yet marked as classified.
And as for Joe Biden's situation, it appears to have been virtually identical to what happened in the case of Mike Pence's VP-era documents. And since the DOJ has cleared Pence, I suspect that the same thing will happen with respect to Biden's case.
OCU