Are multi-sigs multiple keys signing on a single shared signature, or one signature per key?

Question

And if it is a single shared signature, how does that work?

Answer 1

Multisig is multiple keys with one signature per key.

Shamir's Secret Sharing is how a single key can be split in multiple parts, and those parts combined to recover the key (which can be used to create a signature):

a form of secret sharing, where a secret is divided into parts, giving each participant its own unique part, where some of the parts or all of them are needed in order to reconstruct the secret.

Counting on all participants to combine the secret might be impractical, and therefore sometimes the threshold scheme is used where any k of the parts are sufficient to reconstruct the original secret.

A Github or web search on "shamir secret sharing" will help you find implementations.