Understanding Terraform for_each loop iteration

I am learning terraform and trying to understand the for_each loop iteration in terraform.

I am iterating through a loop for creating RGs in Azure cloud and what I want to understand is the difference between accessing the value of an instance using . or [""]. So for example, below is my tfvar file:

resource_groups = {
  resource_group_1 = {
    name     = "terraform-apply-1"
    location = "eastus2"
    tags = {
      created_by = "vivek89@test.com"
    }
  },
  resource_group_2 = {
    name     = "terraform-apply-2"
    location = "eastus2"
    tags = {
      created_by = "vivek89@test.com"
    }
  },

  resource_group_3 = {
    name     = "terraform-apply-3"
    location = "eastus2"
    tags = {
      created_by = "vivek89@test.com"
      contact_dl = "vivek89@test.com"
    }
  }
}

and below is my terraform main.tf file:

resource "azurerm_resource_group" "terraformRG" {
  for_each = var.resource_groups
  name = each.value.name
  location = each.value.location
  tags = each.value.tags
}

I am confused with the expression in for_each in RG creation block. Both the below codes works and create RGs:

name = each.value.name
name = each.value["name"]

I want to understand the difference between the two and which one is correct.