AWS S2S VPN - Route Based Network CIDR Blocks

Hello.

Remote and Local IPv4 network CIDR's

What you want to know is the CIDR set on the screen below in the AWS management console?

If so, please set the on-premises CIDR for "Local IPv4 network CIDR".
There is no problem if you set the AWS VPC CIDR for "Remote IPv4 network CIDR".
This is a CIDR setting that allows the VPN tunnel to pass through, so if you set it to 0.0.0.0/0, all communications will be able to pass through.
If there is no need to limit it, I think you can set it to 0.0.0.0/0.