How to use template with references to script in another repository within Azure DevOps Pipelines?

If your pipeline has templates in another repository, or if you want to use multi-repo checkout with a repository that requires a service connection, you must let the system know about that repository. The repository keyword lets you specify an external repository.

You could refer to Repository resource and templates in another repository for more details.

Such as in templates repository, we'd expect to find test.yml at the root.

resources:
  repositories:
  - repository: templates
    type: git
    name: templates
steps:
- template: test.yml@templates

Update1

I update your code and it works, please check it.

The repo HelloWord structure and the repo contain template file and PS1 file

Template #1 with reference to local script. Note: I add checkout step to checkout the HelloWord and pipeline self repo

  parameters:
  - name: azureSubscription
    type: string
  - name: name
    type: string
  
  jobs:
  - job: HelloWorld
    displayName: Hello World
    variables:
      name: ${{ parameters.name }}
    steps:
    - checkout: self
    - checkout: helloworld
    - task: AzurePowerShell@4
      name: HelloWorld
      displayName: Hello World
      inputs:
        azureSubscription: ${{ parameters.azureSubscription }}
        scriptType: filePath
        scriptPath: $(build.sourcesdirectory)/Helloworld/Subfolder/HelloWorld.ps1
        azurePowerShellVersion: latestVersion
        failOnStandardError: true
        scriptArguments:
          -Name "$(name)"

In the another repo create yaml build definition.

resources:
  repositories:
    - repository: helloworld
      type: git
      name: HelloWorld
pool:
  vmImage: 'windows-latest'
 
stages:
 
- stage: Variables
  jobs:
  - template: hello-world.yml@helloworld
    parameters:
      azureSubscription: 'My Subscription'
      name: 'Billy'

Result: