How to get the list of subnets in Terraform

Question

I'm new in terrform and I spent too much time for searching how to get the list of the subnets.
I have two modules : VPC and subnet.
I successfully created the VPC and the subnet resources.
Then, I created a route table resource for the VPC that takes as attribute the vpc_id.
After that I need to link this route table by assigning values to two attributes : subnet_id and route_table_id.
So, I have one VPC that contains 4 subnets in a specific AZ.
When I created the 4 subnets, I used the for_each attribute to make things dynamic.
Now, I would like to create route table link resources to link the 4 subnets that I have created with the route table. So I need to get the list of the subnet created and use the for_each attribute.
Here is the code
main.tf:

module "vpc_fondation_services" {
  source                      = "./modules/vpc"
  vpc_name                    = "vpc_services"
  ip_range                    = "10.221.0.128/26"
}

module "subnet_nominal" {
  source                      = "./modules/subnet"
  for_each                    = local.subnets_nominal
  subnet_name                 = each.key
  ip_range                    = each.value
  subregion_name              = local.nominal_az
  net_id                      = module.vpc_fondation_services.net_id
}
module "route_table_vpc_fondation_service" {
    source                    = "../network_security/modules/route_table"
    route_table_name          = "route_table_vpc_fondation_service"
    net_id                    = module.vpc_fondation_services.net_id
    
}

resource "outscale_route_table_link" "route_table_link" {
  for_each                    = toset([ for subnet in module.subnet_nominal : subnet])
  subnet_id                   = each.value.subnet_id
  route_table_id              = module.route_table_vpc_fondation_service.route_table_id
}

main.tf of the subnet module

resource "outscale_subnet" "subnet" {
  net_id         = var.net_id
  ip_range       = var.ip_range
  subregion_name = var.subregion_name
  tags {
    key          = "Name"
    value        = var.subnet_name
  }
}

outputs.tf of the subnet module

output "ip_range" {
  value = outscale_subnet.subnet.ip_range
}

output "subnet_id" {
  value = outscale_subnet.subnet.subnet_id
}

output "state" {
  value = outscale_subnet.subnet.state
}

output "available_ips_count" {
  value = outscale_subnet.subnet.available_ips_count
}

the variables.tf of the subnet module

variable "net_id" {
    description = "The ID of the Net for which you want to create a Subnet"
    type        = string
}

variable "subnet_name" {
  description = "The name of Subnet"
  type        = string
}

variable "subregion_name" {
  description = "The AZ where subnet need to be"
  type        = string
}