how to use the DeployAVDSessionHostReplacer.bicep for deployment

I'm trying to use the bicep for deployment, since i need to add some custom code for running custom scripts, as well as deploying other vm extensions on the host.

there isn't much documentation on this, other than the readme, which i tried, but got errors:

| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| 12:34:24 - Error: Code=InvalidTemplate; Message=Deployment template
| validation failed: 'The template resource 'deployFunctionApp' at line
| '390' and column '9' is not valid: The language expression property
| array index '2' is out of bounds.. Please see
| https://aka.ms/arm-functions for usage details.'.

in the deploy/bicep/DeployAVDSessionHostReplacer.bicep
it calls
module deploySessionHosts 'modules/AVDStandardSessionHost.bicep' = [for vm in VMNames: {

which references
deployStandardTemplateSpec.bicep
../../../StandardSessionHostTemplate/DeploySessionHosts.json

What steps am i missing to deploy the bicep, instead of the arm?