You can choose "Download Devices" to get all of them in a .csv file and sort on operatingSystemVersion. (Windows 7 is NT 6.1)
(
https://portal.azure.com/#blade/Microsoft_AAD_Devices/DevicesMenuBlade/Devices/menuId/)
Or you can connect to AzureAD (Connect-AzureAD) and use this Powershell command to filter all OS versions containing 'NT' (Not sure if that works, don't have any Windows 7 machines in our Azure AD :) If it doesn't work, remove " Where-Object DeviceOSVersion -match 'NT' " to see all machines and verify what the correct syntax should be for the -match 'xx' )
Get-AzureADDevice | Where-Object DeviceOSType -eq 'Windows' | Where-Object DeviceOSVersion -match 'NT' | Select-Object DisplayName, DeviceOSType,DeviceOSVersion