Hi @Raul_Lorca and welcome to the Community Forum! Unfortunately, as it stands it is not possible to see subtasks in the Board itself; subtasks are only available when you click on a card to open the tasks details. I would recommend voting on this thread Subtasks in Board View regarding the display of subtasks in Board project; although I can not promise when this will potentially be implemented.
If you’re looking to see all tasks from multiple projects into one Board, you can create a “Master” Board project and multihome all tasks from your project into this Master project. But once again, this won’t work for subtasks.
My apologies for not being of a greater help here. Let me know if you have any follow-up questions.