Retrieve IDs in Terraform before script execution

I'm a freshman and I don't know if I have a general thinking error but how do I get around the issue that when I want to roll out a script that there are some IDs missing that I would only get with splitting my project. Right now I'm stuck at the nat gateway creation which requires the subnet ID. But the subnets are created in the same vpc.tf and I dont know if it is meant to be used in different files.

Here's my script so far. Thanks for your answers

terraform {
  required_providers {
    aws = {
      source = "hashicorp/aws"
      version = "5.1.0"
    }
  }
}

provider "aws" {
  region = "eu-central-1"
}

resource "aws_vpc" "main" {
  cidr_block = "172.31.0.0/16"
  tags = {
    Name = "main-vpc"
  }
}

resource "aws_subnet" "presentationtier-a" {
  vpc_id     = aws_vpc.main.id
  cidr_block = "172.31.1.0/27"
  map_public_ip_on_launch = "true"
  availability_zone = "eu-central-1a"
  tags = {
    Name = "presentationtier-subnet-a"
  }
}

resource "aws_subnet" "apptier-a" {
  vpc_id     = aws_vpc.main.id
  cidr_block = "172.31.2.0/27"
  availability_zone = "eu-central-1a"
  tags = {
    Name = "apptier-subnet-a"
  }
}

resource "aws_subnet" "datatier-a" {
  vpc_id     = aws_vpc.main.id
  cidr_block = "172.31.3.0/27"
  availability_zone = "eu-central-1a"
  tags = {
    Name = "datatier-subnet-a"
  }
}

resource "aws_subnet" "datatier-b" {
  vpc_id     = aws_vpc.main.id
  cidr_block = "172.31.4.0/27"
  availability_zone = "eu-central-1b"
  tags = {
    Name = "datatier-subnet-b"
  }
}

/*
Elastic IP
resource "aws_eip" "vpc" {
  domain   = "vpc"
}

// I created it through the AWS console so it wouldn't get removed with 'terraform destroy' and to keep the ID for the nat gateway.
*/


resource "aws_nat_gateway" "main" {
  allocation_id = "eipalloc-0..."
  subnet_id = ""
  tags = {
    Name = "gw NAT"
  }

  # To ensure proper ordering, it is recommended to add an explicit dependency
  # on the Internet Gateway for the VPC.
  # depends_on = [aws_internet_gateway.example]

}

aws_subnet returns the id of a subnet. So you use that id. For example, to use subnet apptier-a:

subnet_id = aws_subnet.apptier-a.id