Finding data using MySQL queries in the Node tables
The concept that you are missing is that field data can be on any Drupal entity, nodes, users, etc.  This means that when you JOIN, you need to add in some additional clauses.  In particular, you need to check the entity type in the field data, as the entity ID isn't unique across entity types.  You also need to check that the field value isn't deleted.  So, you would have something like

LEFT JOIN field_data_field_usted_es p
  ON p.entity_id = u.uid AND p.entity_type = 'user' AND p.deleted = 0


Those three columns are typically all on need to JOIN ON.  The rest of the columns would be what you return in the query, or use in a WHERE.

Most of the time, it is easiest to build up a query with Views, preview the SQL, and then use that as your starting point for a query outside of Drupal.

In your case, you could also use Views Data Export to generate your CSV inside Drupal itself.