You have multiple Azure VNETs where each VNET is peered with all other VNETs. You deploy Azure Bastion to one of the VNETs. What VMs will be accessible using Azure Bastion?

If all VNETs are peered, then VMs in all VNETs will be available through Azure Bastion.

Azure Bastion allows you to access VMs on peered VNETs as well, not just those VMs that are in the same VNET that the Azure Bastion is deployed in.