Checkout Field API for Drupal 8
It has several functions implemented and hook_entity_bundle_field_info might be what you need, here is an example of textfield from the docs of that hook
function hook_entity_bundle_field_info(\Drupal\Core\Entity\EntityTypeInterface $entity_type, $bundle, array $base_field_definitions) {
// Add a property only to nodes of the 'article' bundle.
if ($entity_type->id() == 'node' && $bundle == 'article') {
$fields = array();
$fields['mymodule_text_more'] = BaseFieldDefinition::create('string')
->setLabel(t('More text'))
->setComputed(TRUE)
->setClass('\Drupal\mymodule\EntityComputedMoreText');
return $fields;
}
}
You might also need Field Storage, checkout hook_entity_field_storage_info