There are cases if you want to start/stop all VMs in particular resource group in parallel within Azure. You can set it up with Automation using Scheduled actions. Other way which you can do by using PowerShell or Azure CLI
If you are using PowerShell, simply you can do
Get-AzVm -ResourceGroupName 'MyResourceGroup' | Start-AzVM
If you are using Azure CLI/Bash
az vm start --ids $(az vm list -g MyResourceGroup --query ".id" -o tsv)
Where MyResourceGroup is the name of your ResourceGroup. Happy Azurifying!