sample.dart

 // (6) set value to drop down here
dev.log('goto page with docId = $value');
snapshot.data!.docs.where((e) => e.id == value).forEach((e) {
  dev.log('${e.data()}');
  setState(() {
    dropdownValue = value.toString();
    // set your value here
    dev.log(e["timestamp"]);
  });
});