Not sure why this does what it does

weird.tf

###########################################################
# Setup
###########################################################

terraform {
  required_version = ">= 0.15"
  experiments = [
    module_variable_optional_attrs
  ]
}

###########################################################
# Some variables to act as defaults
###########################################################

variable "default_string" {
  default = "string"
  type    = string
}

variable "default_number" {
  default = 42
  type    = number
}

variable "default_list" {
  default = []
  type    = list(string)
}

variable "default_map" {
  default = {}
  type    = map(string)
}

###########################################################
# Var representing an INPUT as from a module's output or
# another source (it doesn't matter)
###########################################################

variable "input" {
  type = map(object({
    string = optional(string)
    number = optional(number)
    list   = optional(list(string))
    map    = optional(map(string))
  }))

  default = {
    foo = {
      number = 99
      map = {
        bingo = "bango"
      }
    }
  }
}

###########################################################
# Synthesis
###########################################################

locals {

  ######################
  # Collect the defaults
  defaults = {
    string = var.default_string
    number = var.default_number
    list   = var.default_list
    map    = var.default_map
  }

  input = { for name, overrides in var.input:
    name => {
      for attr, obj in overrides:
        attr => obj
        if obj != null
    }
  }

  ######################
  # Combine defaults with INPUT
  combined = {
    for name, overrides in local.input:
      name => merge(local.defaults, overrides)
  }
}

######################
# RESULT
output "combined" {
  value = local.combined
}

so weird