What is the use of the service VPC?

Vpc allows you to create private ,isolated virtual network with in public cloud

By default, how many networks do we have and into how many subnets are they divided in VPC service?

What is the use of the service VPC?

How is the IPv4 architecture different from IPv6 architecture and how is IPv6 advantageous or disadvantageous?

Why is subnetting actually necessary? Can't we just assign another network instead of dividing a network?