How to set a default account for multiple aws providers in terraform?

I need to create resources for multiple accounts in my terraform code. So, I have created multiple providers in aws and using the same for individual modules. Other than above, if I create any resource, I want to use one of the accounts as default.

I tried below code but not working.

provider "aws" {
  alias  = "silver"
  region = "us-east-2"
  profile = var.profile
}

provider "aws" {
  alias  = "gold"
  region = "us-east-2"
  profile = "gold"
}
provider "aws" {
  alias  = "plt"
  region = "us-east-2"
  profile = "plt"
}
provider "aws" {
  alias  = "infra"
  region = "us-east-2"
  profile = "infra"
}
provider "aws" {
  alias  = "sandbox"
  region = "us-east-2"
  profile = "sandbox"
  default = true
}

Even I tried below setting but getting syntax error.

providers = { 
aws.infra = "aws.infra" 
aws.sandbox = "aws.sandbox" 
aws.plt = "aws.plt"
aws.gold = "aws.gold"
default = "aws.sandbox" 
 }

I tried below code also but I am getting syntax error.

providers = { 
aws = aws.sandbox 
 }

below is the error

╷ │ Error: Unsupported argument │ │ on c1-versions.tf line 52: │
52: providers = { │ │ An argument named "providers" is not expected here.

The default configuration for a given provider is written as just the provider's local name without any alias part.

For example, if you want a child module to treat aws.sandbox from the root module as its default configuration for the provider named aws then you can write that like this:

providers = {
  aws = aws.sandbox
}

The above tells Terraform that the default configuration for the provider inside the child module is the same configuration as the one named "sandbox" in the calling module. Each module has its own set of provider configuration identifiers, so you can use any combination of provider configurations from the parent module to populate the provider configuration addresses required by the child module.