A Google document contains a dropdown; the values in the dropdown are Sheet names for a given Google Sheet (spreadsheet).

You want to know:

• is there a way to allow a script to scan a drop-down on a Google Document
• can a Sheet name selected in a Google Docs dropdown be used to execute a routine on the relevant Sheets-Sheet.

About Google Docs dropdowns

It is worth noting that the options for a dropdown in Google Docs can't be referenced from a range. To the best of my knowledge, they must be entered manually.

Is there a way to allow a script to scan a drop-down on a Google Document?

No.

The only triggers available to Google Docs are:

• onOpen: when a user opens a document
• onInstall: when a user installs an Editor add-on
• time-driven: lets a script execute at a particular time or on a recurring interval.

There is no equivalent of onEdit which is available in Google Sheets. The result is that there is no way to automatically trigger a script based on selecting a value from the dropdown.

Can a Sheet name selected in a Google Docs dropdown be used to run a routine on the relevant Sheets-Sheet

Yes.

Though there is no automatic trigger to run a script in a Google Document, it IS possible to manually run a script that will take a value from a Docs dropdown and run a routine on a given sheet on a given Google spreadsheet.

A method is described in the answer by @qwertzguy in How to get in Apps Script the value of a dropdown in a Google Doc?.

The following script (bound to the Google Doc) demonstrates the method.

function applyDocDropdown() {
  var doc = DocumentApp.getActiveDocument()
  var id = doc.getId()
  var url = "https://docs.google.com/feeds/download/documents/export/Export?exportFormat=docx&id=" + id
  var blob = UrlFetchApp.fetch(url, { headers: { authorization: "Bearer " + ScriptApp.getOAuthToken() } }).getBlob()
  blob.setContentType("application/zip")
  const content = Utilities.unzip(blob);
  let xml = ""
  for (let file of content) {
    if (file.getName() == "word/document.xml") {
      xml = file.getDataAsString()
    }
  }
  var dropDownValues = XmlService.parse(xml)
    .getRootElement()
    .getDescendants()
    .filter(e => e.getType() == "ELEMENT")
    .filter(e => e.asElement().getName() == 'dropDownList')
    .map(e => ({
      type: e.getParentElement().getChildren()[0].getAttributes()[0].getValue(),
      currentValue: e.asElement().getAttributes()[0].getValue()
    }))
  
  // display all results
  Logger.log(dropDownValues) // DEBUG

  // display results from dropdown#1
  Logger.log(dropDownValues[0]) // DEBUG

  // get the dropdown value in dropdown#1
  var dropdownSheetName = dropDownValues[0].currentValue
  Logger.log("DEBUG: the DOC dropdown sheet name = "+dropdownSheetName)

  // get the sheet
  var ssId = "<<insert spreadsheet id>>"
  var ss = SpreadsheetApp.openById(ssId);
  var sheet = ss.getSheetByName(dropdownSheetName)
  Logger.log("DEBUG: this is spreadsheet sheet named: "+sheet.getName())
  var range = sheet.getRange(1,1)
  var value = range.getValue()
  Logger.log("DEBUG: the value of cell "+range.getA1Notation()+"  = "+value)
}

Script results

[{type=Sheet names, currentValue=Staff}, {currentValue=Option 1, type=my second dropdown}]
{currentValue=Staff, type=Sheet names}
DEBUG: the DOC dropdown sheet name = Staff
DEBUG: this is spreadsheet sheet named: Staff
DEBUG: the value of cell A1 = this is Staff

Note: Cell A1 in each sheet contained a text value = "this is "+sheetname

SAMPLE - Document

SAMPLE - Dropdown: "Sheet names"

SAMPLE - Dropdown: "my second dropdown"