Well, in crypto all your assets are on the blockchain. But you can access them using wallet software. You can have multiple addresses that have a balance, and most of the time anyone can see those balances, as the ledger is synchronized and transactions are checked by all clients.

However, in order to access and send the money, you will need proof that you are the owner. Therefore every address has a keypair. In older desktop clients, there was a local file called wallet.dat which stored all of the owned wallet addresses and their private keys. That file could optionally be encrypted. Newer clients often use a mnemonic phrase and derive all keys for the addresses based on that single phrase, but the person in question still had the original wallet.dat, even unencrypted, meaning he could access the keys all along. Not sure what the mnemonic had to do with it. Perhaps that belonged to a newer wallet where he imported the old addresses into.