Hi Devashish!

Auto mode VPC networks are created with one subnet per region at creation time and automatically receive new subnets in new regions. The subnets have IPv4 ranges only, and all subnet ranges fit inside the 10.128. 0.0/9 CIDR block.

VPC network overview | Google

https://cloud.google.com/vpc/docs/vpc#:~:text=Auto%20mode%20VPC%20networks%20are%20created%20with%20one%20subnet%20per,0.0%2F9%20CIDR%20block.