Given the name of an Azure App Service, I want to be able to find the name of the App Service plan. I've inspected the object returned by Get-WebApp but it doesn't contain any reference to the App Service Plan used.
you can use this bit:
Get-AzWebApp | Select-Object name, serverfarmid
this will return Web App name and the Service Plan Id.
( Get-AzWebApp | Select-Object -ExpandProperty ServerFarmId ) -split '/' | Select-Object -Last 1