Use the Get-ChildItem cmdlet with the -Recurse switch: Scenario 1:
Example
Get-ChildItem -Path c:\test -Filter File-to-copy.ps1
Copy and Try it
Get-ChildItem -Path c:\powershell-scripts -Filter File-in-subfolder.ps1
Get-ChildItem -Path c:\powershell-scripts -Filter File-in-subfolder.ps1 -Recurse