Short answers, pulled from the story.
Whitfield Diffie and Martin Hellman first described the notion of a digital signature scheme in 1976, though they only conjectured that such schemes existed. Ronald Rivest, Adi Shamir, and Len Adleman then invented the RSA algorithm, which could produce primitive digital signatures.
Lotus Notes 1.0, released in 1989, was the first widely marketed software package to offer digital signatures. It used the RSA algorithm.
A digital signature is mathematically bound to the content of the specific message it signs, making it infeasible to copy it onto a different document. A handwritten signature can be physically or digitally copied onto forged documents.
It is the accepted security standard for digital signature schemes, first rigorously defined by Shafi Goldwasser, Silvio Micali, and Ronald Rivest in 1988. It requires that generating a valid signature without knowing the signer's private key be computationally infeasible, even when an attacker can request signatures on messages of their own choosing.
Hashing a message before signing makes the process faster, ensures compatibility across different input types, and preserves message integrity by allowing the recipient to detect if any blocks are missing or out of order.
They are quantum-resistant digital signature schemes. CRYSTALS-Dilithium and Falcon are based on hard mathematical problems in lattices, while SPHINCS+ is based on hash functions. All three are designed to remain secure against attacks from quantum computers.