If you are not familiar with modules, check out Ansible - Getting Started with Modules.

Prerequisites

• Before you can use the Ansible Amazon Web Services (AWS) modules, you will need to install the AWS CLI tool on the hosts that will be using the Ansible Amazon Web Services (AWS) modules. Check out my article on Getting Started with the Ansible Amazon Web Services (AWS) modules.
• You will also need to set your Amazon Web Services (AWS) Profile Configurations. Check out my article Set Amazon Web Services (AWS) Profile Configurations.
• The aws_s3_bucket_info requires the following packages. Check out my article Resolve "boto3 required for this module".
  • botocore version 1.25.0 or higher
  • boto3 version 1.22.0 or higher
• Python 3.6 or higher must be used. The ansible --version command can be used to list the version of Python being used with Ansible. If your Ansible installation is used a version lower than Python 3.6, one solution would be to install Ansible in a Python virtual environment using Python 3.6 or higher.
• The amazon.aws collection will need to be installed. Check out my article on Install a collection using the ansible-galaxy collection install command.

ec2_vpc_subnet_info can be used to list your Amazon Web Services (AWS) Virtual Private Cloud (VPC) Subnets.

---
- name: main play
  hosts: localhost
  tasks:
  - name: list AWS VPC Subnets
    amazon.aws.ec2_vpc_subnet_info:
    register: aws_ec2_vpc_subnets

  - debug:
      var: aws_ec2_vpc_subnets
...

Something like this should be returned.

ok: [localhost] => {
    "aws_ec2_vpc_subnets": {
        "changed": false,
        "failed": false,
        "subnets": [
            {
                "assign_ipv6_address_on_creation": false,
                "availability_zone": "us-east-1d",
                "availability_zone_id": "use1-az6",
                "available_ip_address_count": 4091,
                "cidr_block": "172.31.32.0/20",
                "default_for_az": true,
                "enable_dns64": false,
                "id": "subnet-01234417780abcdbc",
                "ipv6_cidr_block_association_set": [],
                "ipv6_native": false,
                "map_customer_owned_ip_on_launch": false,
                "map_public_ip_on_launch": true,
                "owner_id": "713542074252",
                "private_dns_name_options_on_launch": {
                    "enable_resource_name_dns_a_record": false,
                    "enable_resource_name_dns_aaaa_record": false,
                    "hostname_type": "ip-name"
                },
                "state": "available",
                "subnet_arn": "arn:aws:ec2:us-east-1:72345207233:subnet/subnet-01234417780abcdbc",
                "subnet_id": "subnet-01234417780abcdbc",
                "tags": {},
                "vpc_id": "vpc-011234cfa335abcd1"
            }
        ]
    }
}

Almost always, I end up creating a list that contains each Subnet ID.

---
- name: main play
  hosts: localhost
  tasks:
  - name: amazon.aws.ec2_vpc_subnet_info
    amazon.aws.ec2_vpc_subnet_info:
    register: aws_ec2_vpc_subnets

  - name: append each Subnet ID to the subnet_ids list
    set_fact:
      subnet_ids: "{{ subnet_ids | default([]) + [ item.id ] }}"
    with_items: "{{ aws_ec2_vpc_subnets.subnets }}"

  - debug:
      var: subnet_ids
...

Which should return something like this.

ok: [localhost] => {
    "subnet_ids": [
        "subnet-03f11417780f61234",
        "subnet-09b70fa463fcd5678",
        "subnet-0f35c3586e5099012",
        "subnet-03c64e403dc5b3456",
        "subnet-0316e4d9fcd4e7890",
        "subnet-05727079637281234"
    ]
}