Authenticate with Azure, list subscriptions for tenants and select a subscription to work with.

Connect-AzAccount
 
Get-AzSubscription
 
Select-AzSubscription -SubscriptionId <id>

This shows subscription and tenant info for current connections.

Get-AzContext -ListAvailable | fl

Set-AzContext -Tenant "xyz"

Set-AzContext -Subscription "xyz"

Loop over all subscriptions in a tenant and run some command against them.

foreach ($sub in Get-AzSubscription -TenantId "xyz") {
    Write-Host $sub.id
}

Example of getting creation/last update timestamp for all custom policy definitions.

foreach ($sub in Get-AzSubscription -TenantId "xyz") {
    Get-AzPolicyDefinition -SubscriptionId $sub.id | select -ExpandProperty Properties | where {$_.PolicyType -eq "Custom"} | select DisplayName -ExpandProperty Metadata | select DisplayName, createdOn, updatedOn | fl
}

$azResources = Get-AzResource
 
$azResources | foreach {
  $_.ResourceId
}

Get-AzResourceGroup | foreach {Get-AzStorageAccount -ResourceGroupName $_.ResourceGroupName}