I thought it was essentially a list of all the xpubs in the multisig

Xpub and descriptor are different.

High level: for a multisig wallet you need ALL of the public keys for every spend, for a single signature wallet, you only need your private key. For anything more complex than a single signature wallet you need to understand how the wallet was set up. So even if it’s a 2 of 3 that means you only need 2 of the 3 private keys, but you still need all 3 public keys. So in this example you would need all 3 xpubs.

Here is a good primer https://blog.casa.io/introducing-wallet-https://m.youtube.com/watch?v=ePx5lBSI0es