How can I extract a particular worksheet in an xlsx spreadsheet as a text file?

The following snippet might give you a starting point.

You will probably have to find the correct path of the Interop dll on your hardrive and then adjust the add-type cmdlet.

$dir is the directory from which the workbooks are opened and into which they're written.

$dir = convert-path ~/ZZZ/Excel/Export-CSV/

add-type -path 'C:\Program Files (x86)\Microsoft Office\Office16\DCF\Microsoft.Office.Interop.Excel.dll'

$xls = new-object Microsoft.Office.Interop.Excel.ApplicationClass
$xls.visible       = $true
$xls.displayAlerts = $false

foreach ($wbFile in get-childItem $dir\*.xls*) {
   $wb = $xls.workbooks.open($wbFile.fullName)

   try {
      $sh = $wb.sheets('Approval_Logs')
   }
   catch {
      if ($_.exception.message -match 'Invalid index.') {
         write-host "Expected sheet not found in $($wb.name)"
         $wb.close()
         continue
      }
      throw $_
   }

   $sh.select()

   $csvFile = "${dir}$($wbFile.basename).csv"
   $wb.saveAs($csvFile, 6, $false)
   write-host "$csvFile was saved"

   $wb.close()

}

$xls.quit()