i create a custom Entity, 

> CustomEntity.php

    public static function baseFieldDefinitions(EntityTypeInterface $entity_type) {
        $fields = parent::baseFieldDefinitions($entity_type);

        $fields['name'] = BaseFieldDefinition::create('string')
            'max_length' => 50,
            'text_processing' => 0,
          ->setDisplayOptions('view', [
            'label' => 'above',
            'type' => 'string',
            'weight' => -4,
          ->setDisplayOptions('form', [
            'type' => 'string_textfield',
            'weight' => -4,
          ->setDisplayConfigurable('form', TRUE)
          ->setDisplayConfigurable('view', TRUE);

        return $fields;

We can add fields programmatically here for the Main entity and then all bundles ll automatically inherited fields from mother.

So i create a bundle in myModule/config/install:

> myModule.customEntity.bundleA.yml

    uuid: db129d12-849a-4e68-bb56-d787b4b0cf42
    langcode: en
    status: true
    dependencies: {  }
    id: bundleA
    label: bundleA

I am looking for add fields programmatically to that bundle .

So i create with the admin interface all fields and have export all config files into my install folder.
But it's a lot of files: storage / view / field / form.

Is there another way like the main entity for example ?