Populate variable by combining hcl files

I am seeking to split up my terraform.tfvars files into separate *.hcl files that are combined to populate a variable for a module. For reference, I am trying to replicate terraform-google-firewall-rules/README.md at main · r-teller/terraform-google-firewall-rules · GitHub but using HCL instead of JSON. I’ve tried different combinations but have been unable to find an appropriate solution.