Find out all Site Collection Administrators from a Web Application using PowerShell

clear
Remove-PSSnapin Microsoft.SharePoint.Powershell -ErrorAction SilentlyContinue
Add-PSSnapin Microsoft.SharePoint.Powershell

############### VARIABLES ################################

$rootsiteUrl = "https:/webappurl"

################################################################

$rootSite = New-Object Microsoft.SharePoint.SPSite($rootsiteUrl)
$spWebApp = $rootSite.WebApplication
 
foreach($site in $spWebApp.Sites)
{
    foreach($siteAdmin in $site.RootWeb.SiteAdministrators)
    {
        Write-Host "$($siteAdmin.ParentWeb.Url) - $($siteAdmin.DisplayName)"
    }
    $site.Dispose()
}
$rootSite.Dispose()