Craft CMS 3 - Add a Field to a section fields layout programmatically

index.php

<?php

// To put somewhere in a module, plugin or content migration

// Get the entry type to add the new field to
$entryType = Craft::$app->getSections()->getSectionByHandle('someSectionHandle')->entryTypes[0];

$field = Craft::$app->getFields()->getFieldByHandle('someFieldHandle');

// Get current fieldLayout
$fieldLayout = $entryType->getFieldLayout();

$amendedFields = ArrayHelper::merge([$field], $fieldLayout->getFields());
$fieldLayout->setFields($amendedFields);
$fieldLayout->getTabs()[0]->setFields($amendedFields);
            
// Change entryType fieldLayout
$entryType->setFieldLayout($fieldLayout);

// Save entryType configuration
Craft::$app->getSections()->saveEntryType($entryType);