Hyper-V: How do VMs communicate with external?

Simple scenario:

VM --> vNIC --> vSwitch (external) --> physNIC --> physSwitch

The vNIC assigned to the VM has MAC address aa:aa:aa:aa:aa:aa, the physical NIC (physNIC; the vSwitch of type external is connected to it) has bb:bb:bb:bb:bb:bb.

What mechanism ensures that when the VM sends a network packet to the external network (the physical network connected to the physical switch physSwitch), the MAC address of its vNIC (aa:aa:aa:aa:aa:aa) is used, and not the MAC address of the physNIC (bb:bb:bb:bb:bb:bb)? In other words: what makes physSwitch "see" aa:aa:aa:aa:aa:aa when the VM communicates to an external endpoint?