How to produce a map from the cartesian product of two lists to use with for_each on a resource with Terraform

Your loop is being nested.

Use this instead.

# Clusters
variable "datadog_gke_clusters" {
  type = list(object({
    name        = string
    environment = string
    url = object({
      private = string
      public  = string
    })
  }))
  default = [
    # NPRD
    {
      name        = "cluster01"
      environment = "nprd"
      url = {
        private = "https://private.domain-priv.com"
        public  = "https://public.domain.com"
      }
    },
    # PROD
    {
      name        = "cluster02"
      environment = "nprd"
      url = {
        private = "https://private.domain-priv.com"
        public  = "https://public.domain.com"
      }
    }
  ]
}

# Point of views
variable "gke_slo" {
  type = list(object({
    zone              = string
    monitor_threshold = number
  }))
  description = "Response time threshold in ms to respond to a request."
  default = [
    {
      zone              = "private"
      monitor_threshold = 50
    },
    {
      zone              = "public"
      monitor_threshold = 100
    },
  ]
}

locals {
  result = {
    for tuple in setproduct(var.datadog_gke_clusters, var.gke_slo): "${tuple[0].name}_${tuple[1].zone}" => {
      name              = tuple[0].name
      environment       = tuple[0].environment
      url               = tuple[0].url
      zone              = tuple[1].zone
      monitor_threshold = tuple[1].monitor_threshold
    }
  }
}

output "result" {
  value = local.result
}

As in Terraform, a loop "products" a result (a map, a list, an object...), in this case, you have to avoid nested loop:

# Clusters
variable "datadog_gke_clusters" {
  type = list(object({
    name        = string
    environment = string
    url = object({
      private = string
      public  = string
    })
  }))
  default = [
    # NPRD
    {
      name        = "cluster01"
      environment = "nprd"
      url = {
        private = "https://private.domain-priv.com"
        public  = "https://public.domain.com"
      }
    },
    # PROD
    {
      name        = "cluster02"
      environment = "nprd"
      url = {
        private = "https://private.domain-priv.com"
        public  = "https://public.domain.com"
      }
    }
  ]
}

# Point of views
variable "gke_slo" {
  type = list(object({
    zone              = string
    monitor_threshold = number
  }))
  description = "Response time threshold in ms to respond to a request."
  default = [
    {
      zone              = "private"
      monitor_threshold = 50
    },
    {
      zone              = "public"
      monitor_threshold = 100
    },
  ]
}

locals {
  result = {
    for tuple in setproduct(var.datadog_gke_clusters, var.gke_slo): "${tuple[0].name}_${tuple[1].zone}" => {
      name              = tuple[0].name
      environment       = tuple[0].environment
      url               = tuple[0].url
      zone              = tuple[1].zone
      monitor_threshold = tuple[1].monitor_threshold
    }
  }
}

output "result" {
  value = local.result
}

Here we use setproduct to produce a list(list(tuple(cluster,pov)) and then we can iterate on the first list to produce the expected result.

In 2 steps, you can first create your cartesian product in a variable:

locals {
  datadog_gke_clusters_by_pov = flatten([
    for cluster in var.datadog_gke_clusters : [
      for pov in var.gke_slo_ : {
        cluster    = cluster
        pov        = pov
      }
    ]
  ])
}

And then use this new variable in your resource block:

resource "datadog_synthetics_test" "gke-monitoring" {
  for_each = {
    for valuue in local.datadog_gke_clusters_by_pov : "${valuue.cluster.name}-${valuue.pov.zone_name}" => valuue
  }
...
# here you can use each.value.cluster.* and each.value.pov.*
}

Have a good day!