i am not expert here but i believe a multisig wallet isn't really a single collection of addresses derived from randomness but rather addresses with a spend script stipulation that must be satisfied. in that way it is a address created by the software with stipulations for the other keys to participate in signing.

an xpub is an extended public key tied to a private key. there is no private key for a multisig but rather many keys therefore there is not one unifying xpub