How can I use terraform module output values to set variables

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Terraform

I am really new to terraform so please forgive anything I say incorrectly. I come from a normal programming background (C/C++/C#/python etc)

In order to avoid duplication, I want to use a module output value to set a variable value in variables.tf using string interpolation. When I do this and then do 'terraform validate' I get an Error: Variables not allowed.

String interpolation worked fine when I used the output value in a different .tf file to set a resource parameter.

resource "google_storage_bucket_iam_binding" "b-viewer" {
  bucket  = google_storage_bucket.b.name
  role    = "roles/storage.objectViewer"
  members = ["abc${module.my_module.project_number}xyz"]
}

How can I achieve this?

What I have tried:

My terraform structure is:

root/variables.tf
root/module/<module name=""> # this is the module source

The module is called "my_module" and has an output e.g.

output "project_number" {
  value = google_project.project.number
}

In root/variables.tf I have:

variable "my_variable" {
   type = string
   default = "abc${module.my_module.project_number}xyz"
{

but this does not work, it throws and error because I have used the ${module.....} in the default value setting.

I also tried creating a local in variables.tf

locals {
  my_project = module.my_module.project_number
}

and then tried using that in the variable:

default = "abc${local.my_project}xyz"

but that did not work either.

I have tried to find answers to this, of course, but have failed and may be completely missing a point, so I apologise in advance if I am being stupid :/