Drupal 8 multi fields in one fields can Someone explain how can i make one field in Drupal 8 that can contain multiple fields in it... I am not allowed to use Paragraph module. I want to make it full programmatically, By the way i know how to make simple field in Drupal 8 but don't know how to set schema or form form dynamic field that can have multiple fields in it.
 For Example when we want to add some field in user or in node, we go to ~/fields/add-field and then select the field type, after in the field setting i want to have one form element that says add filed and the type of the field. and after saving setting, i will generate the field widget and formater depending on the field type selection that i made before.

Is there a solution for this problem (only programmatically)?
If you prefer another approach, a little more clean and following the Drupal structure, you can use Inline Entity Form to do that.

I don't like Paragraphs and Field Collection for just one reason. They don't follow the way that Drupal storages data in DB. If you open your database and search for the tables that this modules create, you will be able to see some strange patterns.

https://www.drupal.org/project/inline_entity_form