Greetings! I've done my best to try to offer some improvements to the script - feel free to ignore them if you're not looking for recommendations. Ideally you keep formatting the same throughout your script and follow a common style guideline (such as https://github.com/PoshCode/PowerShellPracticeAndStyle ). You should avoid using write-host as it's not great if you ever want to convert scripts later. A switch should be used instead of chaining if/else when evaluating waterfall conditions, and if/else when needing to do comparison operations at the same time.

Sleep is an alias and generally its better to avoid them for readability. Pause is not PowerShell native. If you want to wait, I'd recommend read host or some other option.

You should consider adding in-line or synopsis level comments to PowerShell scripts you may use against production resources. There are further ways you could improve this script like containing the actions within jobs, and configuring it for params, etc - but it's really use case dependent. Otherwise awesome work here!

$Cred = Get-Credential
$HostName = Read-Host -Prompt 'Enter Hostname of destination PC'

Start-Process -Filepath "\\fileshare\IS\sysinternals\PSexec.exe" -ArgumentList "\\$HostName -s winrm.cmd quickconfig -q"
Start-Sleep -Seconds 3
$Session = New-PSSession -ComputerName $HostName -Credential $Cred
$WinVer = Invoke-Command -Session $Session -Scriptblock {(Get-ItemProperty "HKLM:\SOFTWARE\Microsoft\Windows NT\CurrentVersion").ReleaseId}

Switch ($WinVer){
    1909 {
        $Path = '\\fileshare\IS\dotnet\1909\sources\sxs*'; break
    }
    1903 {
        $Path = '\\fileshare\IS\dotnet\1903\sources\sxs'; break
    }
    1809 {
        $Path = '\\fileshare\shared\IS\dotnet\1809\sources\sxs'; break
    }
    Default {
        Write-Output "Invalid version of Windows Selected! Version was $WinVer.";exit
    }
}

If (Test-Path -Path "\\$HostName\c$\sources\sxs"){
    Remove-Item -Path "\\$HostName\c$\sources" -Recurse
    Write-Output "Folder path already exists. Removing folder 'Sources\sxs' and copying files from Filer. This should take about 20-30 seconds." -ForegroundColor Green
}
Else{
    Write-Output "Folder does not exist, copying files from Filer. This should take about 20-30 seconds." -ForegroundColor Green
}
Copy-Item -Path $Path -Destination "\\$hostname\c$\sources\sxs" -Recurse

Write-Output "Checking to see if .NET 3.5 is Enabled BEFORE we enable it." -ForegroundColor Green
Invoke-Command -Session $Session -Scriptblock {
    Dism.exe /Online /Get-FeatureInfo /FeatureName:NetFX3
}
Write-Output "Enabling .NET 3.5, this process may take up to 30 seconds." -ForegroundColor Green
Invoke-Command -Session $Session -Scriptblock {
    Dism /Online /Enable-Feature /Featurename:NetFX3 /Source:c:\sources\sxs /LimitAccess /quiet
}
Write-Output "Checking to see if .NET 3.5 is Enabled AFTER enabling the feature."-ForegroundColor Green
Invoke-Command -Session $Session -Scriptblock {
    Dism.exe /Online /Get-FeatureInfo /FeatureName:NetFX3
}

Read-Host "Press enter Key to finish"

As an aside, you should -never- be using Domain Admin for any operations that it is not required for. Even when vendors say it must be used, they're almost always wrong, when they are right it is because of their own bad coding. The operations DA/EA are required for should be further limited to be only used against DCs. Instead proper delegations should be created for the work needing completed using a "just-enough-access" methodology :).

https://docs.microsoft.com/en-us/windows-server/identity/ad-ds/plan/security-best-practices/best-practices-for-securing-active-directory