Pass an Output Value into a Data Block

I'd like to pass an output value into a data block but don't know how

output "vpce_interface_ids" {
  value = join(",", aws_vpc_endpoint.vpce.network_interface_ids)
}

data "aws_network_interface" "ENI_0" {
  id = ""
}

You don't really need to do that.

If your scope is to pass join(",",aws_vpc_endpoint.vpce.network_interface_ids) as id in datasource you can use a local block or pass it directly

Option 1

locals {
  id = join(",", aws_vpc_endpoint.vpce.network_interface_ids)
}

data "aws_network_interface" "ENI_0" {
  id = local.id
}

Option 2

data "aws_network_interface" "ENI_0" {
  id = join(",", aws_vpc_endpoint.vpce.network_interface_ids)
}