Hi @aricwilisch!
Unfortunately there are a number of different ways to do what you described and it sounds like you’ve inherited a system that already has some decisions made which you may be constrained by. With that said, it’d help if you can share some more details about how that system is structured, and then I or someone else can hopefully offer some specific advice that applies to that context.
The general idea here is that your Terraform configuration will either be managing both the subnets and the instances or it will be managing just the instances and thus need to obtain information about the subnet ids that already exist. Both are possible, but which is appropriate and exactly how to achieve it will depend on how the existing system is set up, whether the subnets you will be working in where already created somehow (Terraform or otherwise), etc.