What's the default weight of a form's submit element?
A submit button appears at the bottom of the form usually because it's simply added last in the form definition.

A submit button can still be placed anywhere and weighted using the #weight property.

The default weight is assigned automatically.

The relevant core code is in 

form_builder

// Assign a decimal placeholder weight to preserve original array order.
if (!isset($element[$key]['#weight'])) {
  $element[$key]['#weight'] = $count/1000;
}
else {
  // If one of the child elements has a weight then we will need to sort
  // later.
  unset($element['#sorted']);
}


If not present the#weight property is calculated dynamically based on the count/item position while looping over the form element set.