The traffic between virtual machines in peered virtual networks uses the Microsoft backbone infrastructure.
https://learn.microsoft.com/en-us/azure/virtual-network/virtual-network-peering-overview ExpressRoute private peering supports connectivity between multiple virtual networks. Although this behavior happens by default when linking virtual networks to the same ExpressRoute circuit, Microsoft doesn't recommend this solution. To establish connectivity between virtual networks, VNet peering should be implemented instead for the best performance possible.
https://learn.microsoft.com/en-us/azure/expressroute/virtual-network-connectivity-guidance