Question about azure powershell, defining a veriable twice, and disk creation - powershell

Question

I'm currently studying for az-104 azure exam and going through some scripting exercises.
I can confirm the script below works, but I don't understand how...
in the last few lines of the script the variable $vm is defined twice. how is this possible?
also when you define a variable is it actually running the commands that are being defined? i didn't realize that was the case, but it definitely seems to be.. can someone please explain?
$resourcegroup = "2019dc_group"
$machinename = "2019dc"
$location = "east us"
$storagetype = "Standard_LRS"
$name = "newdisk"
$size = 20
$datadiskconfig = new-azdiskconfig -SkuName $storagetype -location $location -createoption empty -DiskSizeGB $size
$datadisk01 = new-azdisk -diskname $name -disk $datadiskconfig -ResourceGroupName $resourcegroup
$vm = Get-AzVM -name $machinename -resourcegroupname $resourcegroup
$vm = add-azvmdatadisk -vm $vm -name $name -createoption attach -ManagedDiskId $datadisk01.id -lun 1
update-azvm -vm $vm -resourcegroupname $resourcegroup

Answer 1

Variable types in PowerShell are dynamic. They are automatically "adjusted" to the type of the object they are assigned to.
Yes, the commands are being executed first and then the object is placed in the variable. This is also why you are able to use the vm variable in the command and assign the result back to that variable. When the "add-azvmdatadisk" command is executed, the type of the variable is still an Azure VM. When the assignment takes places, it is an Azure VM Data Disk
You can use Get-Member to get the type, methods, properties, events etc. for any object.
$a = 1 #System.Int32
$a | Get-Member
$a = "1" #System.String
$a | Get-Member
$a = Get-Service | Select-Object -First 1 #System.ServiceProcess.ServiceController
$a | Get-Member

Answer 2

The $VM variable is not defined twice. It's being assigned a value twice. One doesn't need to define variables in Powershell, assignment will automatically create one.
Let's review the code. First off,
$vm = Get-AzVM -name $machinename -resourcegroupname $resourcegroup
After the Get-AzVM, you'll have its output in $vm. Nothing fancy here. Note that before this statement, $vm didn't exist, and trying to work with it would generate an error about using null value.
$vm = add-azvmdatadisk -vm $vm -name $name -createoption attach -ManagedDiskId $datadisk01.id -lun 1
Here $vm is used twice. The first usage is when you pass it as an argument to Add-AzVmDataDisk. The cmdlet returns a PSVirtualMachine object, so after adding the disk, you'd get an updated version of $vm into the variable. This might or might not be important a difference. If the Azure VM objects are lazy evaluated, the old $vm would contain hardware information without the new disk. This often is the case, as it improves performance. By assigning the output of the later cmdlet into $vm, you are sure to have up-to-date version of your VM.
If you have a lab environment, try printing $vm before the last disk addition cmdlet call and after it. See what's different on the outputs.