This is actually quite easy but it should be done in the correct way.
You can ignore all this and go and look at the official docs here: https://www.drupal.org/docs/drupal-apis/entity-api/working-with-the-entity-api
And that will get you started, but the tricky bit is then setting the fields. You can set the values easily:
$node_storage = \Drupal::entityTypeManager()->getStorage('node'); // Create a new unsaved node. $node = $node_storage->create([ 'type' => 'page', 'title' => 'Test page', 'field_boolean' => [ 'value' => 0 ], 'moderation_state' => [ 'value' => 'published' ], ]); // Save it. $node->save(); // Get the node ID (only available after save). $nid = $node->id();
But it’s not always easy to figure out what the values should be. For instance, if you’re referencing an entity then you need:
... 'field_related_page' => [ [ 'target_id' => 123 ], ], ...
Remember that creating a field to only have one value doesn’t change how it’s stored. So this means that you’ll need to create a single element array containing the array that has the value for the field in it.
The easiest way to figure this out is to use Drush.
First of all create a new node in the admin backend that you can use as an example and then use Drush to get a PHP command line with Drupal loaded:
$ drush php:cli Psy Shell v0.10.6 (PHP 7.3.23 — cli) by Justin Hileman
Then add a use statement to get to the Node class and load your example node:
>>> use \Drupal\node\Entity\Node; >>> $n = Node::load(456);
Now you can get the fields in their array format to see what it is you need to do use to set a field value:
>>> $n->get('moderation_state')->toArray(); => [ "value" => "published", ]
As I mentioned above, a field value is actually an array of values of the field. So the
 above gets the first value for the moderation state field.