How to identify user who is using iOS / Android Outlook App

You can try below script and this script will correlate with username, license and mobile device

$cred = Get-Credential

#connecting Exchange Online 
$exchangeSession = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri "https://outlook.office365.com/powershell-liveid/" -Credential $cred -Authentication "Basic" -AllowRedirection
Import-PSSession $exchangeSession -DisableNameChecking

#conneting Msolservice
Connect-MsolService -Credential $cred

#Don't forget to Provide correct path
$LicenseMappingPath = "C:\Licmap.txt"
$content = ( Get-Content $LicenseMappingPath | Out-String )
$licname = ( Invoke-Expression $content )

#get user details with license
$j=1
$user= Get-MsolUser -all |Where-Object { $_.isLicensed -eq $true } |select UserPrincipalName,Licenses
$Userdetails = $user |% {
     
     Write-Progress -Activity "Get Users" -Status "$j% User:"
    $lic=($_.Licenses.AccountSkuId)
    $lname= @()
    foreach ($l in $lic) 
    {
        $l2=$l.Split(":")[1]
        $Licn=$licname.Item($l2) 
        $lname +=$Licn
        $Licn =$null
    }
    New-Object -TypeName PSObject -Property @{
          username = $_.UserPrincipalName
          Licence = $lname -join ","
           }
    $lname = $null
    $j++
}|select username,Licence

#Get Mobile device with license details
$i=1
$Userdetails | % {
    $Userdetail = $_
    $mobiledetails= Get-MobileDevice -Mailbox $_.username -ErrorAction ignore | select UserDisplayName,DeviceOS
    if($mobiledetails.DeviceOS -ne $null)
    {
    Write-Progress -Activity "Get Mobile details" -Status "$i% Mobile Device:"
      New-Object -TypeName PSObject -Property @{
          UserPrincipalName = $Userdetail.username
          Licensename = $Userdetail.Licence
          Devicetype = $mobiledetails.DeviceOS
          UserDisplayName = $mobiledetails.UserDisplayName
           }
           $i++
     }

} |select UserPrincipalName,Licensename,Devicetype,UserDisplayName |Export-Csv -Path "c:\Exportmobildetails.csv" -NoTypeInformation

Note: Before run the script change following vlaue  in the script

 $LicenseMappingPath 
-Path "c:\Exportmobildetails.csv"