How to reference a logical id created by ForEach using ampersand from another ForEach in Cloudformation

To reference logical IDs created using the ampersand syntax in a Fn::ForEach loop from another Fn::ForEach loop, you need to use a combination of Fn::Sub and the ampersand syntax. However, there's a small adjustment needed in your code.

In your second Fn::ForEach loop, you're trying to reference the subnet created in the first loop. The correct way to do this is to use Fn::Sub with the ampersand syntax, but without nesting it inside a !Ref. Here's the corrected version:

'Fn::ForEach::PubRtbSubnetsAssoc':
    - CIDR
    - !Ref PublicSubnetCidrs  
    - 'RtbSubAssoc&{CIDR}':
        Type: 'AWS::EC2::SubnetRouteTableAssociation'
        Properties:
            RouteTableId: !Ref PublicRouteTable
            SubnetId: 
                Fn::Sub: 'Cidr&${CIDR}'

In this corrected version, we're using Fn::Sub to construct the logical ID of the subnet created in the first loop. The '&' is included inside the string passed to Fn::Sub, and we use '${CIDR}' instead of '&{CIDR}' because we're already inside a Fn::Sub context.

This syntax allows you to reference the dynamically created logical IDs from the first Fn::ForEach loop in the second loop. The ampersand syntax in Fn::ForEach is specifically designed to allow non-alphanumeric characters in the Collection to be used in the OutputKey parameter, which is useful when you need to reference these resources later.

Remember that when using Fn::ForEach, you need to apply the AWS::LanguageExtensions transform to your template for it to work correctly.
Sources
Fn::ForEach - AWS CloudFormation
Examples of Fn::ForEach in the Conditions section - AWS CloudFormation