Availability Zones represented by a particular letter can mean different AZs to different AWS accounts. For example for Account A us-west-1a can be different from what us-west-1a is for account B.

In Cloudformation, you can use

PublicSubnet1:
    Type: AWS::EC2::Subnet
    Properties:
      VpcId: !Ref VPC
      AvailabilityZone: !Select [ 0, !GetAZs '' ] 
      CidrBlock: !Ref PublicSubnet1CIDR
      MapPublicIpOnLaunch: true
      Tags:
        - Key: Name
          Value: !Sub ${EnvironmentName} Public Subnet (AZ1)

PublicSubnet2:
    Type: AWS::EC2::Subnet
    Properties:
      VpcId: !Ref VPC
      AvailabilityZone: !Select [ 1, !GetAZs  '' ]
      CidrBlock: !Ref PublicSubnet2CIDR
      MapPublicIpOnLaunch: true
      Tags:
        - Key: Name
          Value: !Sub ${EnvironmentName} Public Subnet (AZ2)

Note how different array indexes are chosen for the two Public Subnets in the AvailabilityZone: !Select [ 0, !GetAZs '' ] statement in the Cloudformation template snippet above, thereby ensuring the two subnets are created in two different AZs

I see a similar concept being used in Terraform - https://registry.terraform.io/providers/hashicorp/aws/latest/docs/data-sources/availability_zones

# Declare the data source
data "aws_availability_zones" "available" {
  state = "available"
}

# e.g., Create subnets in the first two available availability zones

resource "aws_subnet" "primary" {
  availability_zone = data.aws_availability_zones.available.names[0]

  # ...
}

resource "aws_subnet" "secondary" {
  availability_zone = data.aws_availability_zones.available.names[1]

...

}