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If they're different per stack, then having them in the stack config file is best.
If they're consistent for all stacks, then putting them in code can simplify things if you want. You can also put them in `Pulumi.yaml`.

If you have another pulumi project that sets up the shared resources (your subnets), then you can use a Stack Reference. That way you don't need to hard code any configs

This sounds like it could also be a good use case for <https://www.pulumi.com/docs/pulumi-cloud/esc/get-started/|Pulumi ESC>. You can centralize any shared configuration in one environment file and have your different environments consume them.