How is a multisig transaction transferred between multiple signers? I'm trying to learn more about doing multisig transactions.
The examples I've seen show the signing of the transaction with 2-of-3, or 3-of-3, where the signatures are added, essentially, at the same time, and same place.
I'm wondering how does one, using rpc or cli, perform a multisig spend from physically separate machines?
How does a signer know that there is a transaction that he/she needs to sign?
Can a 2-of-3 transaction, that initially only contains 1 signature be broadcast, then updated by the 2nd or 3rd signatory?
I'm wondering how does one, using rpc or cli, perform a multi sig spend from physically separate machines?

It is the usually the same exact process, just doing the signing command(s) on multiple devices. The software should be able to handle when it can't fully sign and will produce as many signatures as it can for that transaction.

How does a signee know that there is a transaction that he/she needs to sign?

He has to be given the transaction to sign. If using PSBT, the transaction will usually contain an identifier for the signer that the signer can look for to know whether to sign.

Can a 2-of-3 transaction, that initially only contains 1 sig be broadcast then updated by 2nd or 3rd signatory?

No, the transaction is not fully signed so it cannot be broadcast to the network. It must be given to one of the other signers and they must sign the transaction before it can be broadcast.