The VPN server is configured to provide the client with a list of IP address ranges which should be tunnelled – the rest are automatically excluded and go through your normal route.
For business VPNs, this list is set by your IT team and includes just the company's IP addresses. (These might be public or private addresses; the VPN doesn't care.)
On Windows you can usually use route print
to see which addresses are routed through the VPN's virtual network adapter.
(For commercial VPNs, this list is 0.0.0.0/00.0.0.0/0
and ::/0::/0
, which simply includes everything.)