[reactspa]

However, I couldn't find this signature:

HM7vpPSUbNsfDHRX6gv8xxWcVNHEc/3pOk0YrVehaGoUdbWizznfzOdELkLd1EjSXsW1oE5vHAkNAPzrAVzhuoI=

... anywhere on the blockchain:

https://www.blockchain.com/explorer/search?search=HM7vpPSUbN...

Why not?

Update: I get it now. It doesn't have to have been on the blockchain. It's just a message that was signed with a private key, and...., using Hal Finney's public key (wallet address), the message, and the message signature, we can confirm that it was signed by Hal Finney's private key.