I'm not sure exactly what your code and looks like, so I created the following static approximation from the description:
$data = New-Object PSCustomObject
$data | Add-Member -Type NoteProperty -Name Id -Value 999
$restResponse = New-Object PSCustomObject
$restResponse | Add-Member -Type NoteProperty -Name data -Value $data
Please clarify if this is not a match. You can get the Id value as follows
$restResponse.data.Id
Assign it to another variable
$newVar = "sometext" + $restResponse.data.Id + "moretext"
$newVar
And if your REST response is a collection of data objects, iterate through them
$restResponse.data | Foreach-Object { "sometext" + $_.Id + "moretext" }