Pagination for inline entity form
I'm not ruling out mystery module that I am unaware of,  but I can give some insight. 

The first obvious choice is the API, just reference the hooks in :   /modules/contrib/inline_entity_form/inline_entity_form.api.php

You can also add a FieldWidget in your module like: YOUR_MODULE/src/Plugin/Field/FieldWidget/IEFPagination.php,  then you would select it under Manage Form Display.

namespace Drupal\YOUR_MODULE\Plugin\Field\FieldWidget;

use Drupal\inline_entity_form\Plugin\Field\FieldWidget\InlineEntityFormComplex;

/**
 *
 * @FieldWidget(
 *   id = "ief_paginated",
 *   label = @Translation("IEF - Paginated"),
 *   field_types = {
 *     "entity_reference"
 *   },
 *   multiple_values = true
 * )
 */

class IEFPagination extends InlineEntityFormComplex {

 public function formElement(\Drupal\Core\Field\FieldItemListInterface $items, $delta, array $element, array &amp;$form, \Drupal\Core\Form\FormStateInterface $form_state) {

    $element = parent::formElement($items, $delta, $element, $form, $form_state);

    /* Adjust element here */ 

    return $element;
}

 public static function buildEntityFormActions($element) {

    $element = parent::buildEntityFormActions($element);

    /* Or here */

    return $element;
 }