how to create custom routes ?

azure routes traffic b/w all subnets with in a virtual network, by default. you can create your own routes to override azure's default routing. the ability to create custom routes is helpful if, for example, you want to route traffic b/w subnets through a network virtual appliance (NVA)

Create a route table, create a route, create a virtual network with multiple subnets, associate a route table to a subnet, create an NVA that routes traffic, deploy virtual machines (VM) into different subnets and route traffic from one subnet to another through an NVA.

reference:

https://docs.microsoft.com/en-us/azure/virtual-network/tutorial-create-route-table-portal