The following paragraph under “Digital Signatures using Public Key Cryptography” says:
“This works, but to make it more efficient, instead of signing the entire lengthy message the sender computes a hash (digest) of message and signs that with his public key instead. The receiver can then re-compute the message hash and compare it with the signed hash to ensure that message was not tampered with.”
I believe the sender signs the key with their private key and the receiver will read it with the sender’s public key.