How is Plutus on-chain code able to retrieve and decode datums from UTXOs of
These datums are hardly found in cardano-db-sync or explorers anywhere?
Currently, only datum hashes are able to be a part of the UTXO, so you are only able to verify a datum value on chain. Retrieving the datum itself is therefore on off-chain process.
However CIP-32 is coming very soon (likely end of February/March) and will enable attaching the datum itself to a UTXO, rather than only the datum hash.