magento2 – Adding a customer custom attribute

I have a system where custom attributes were added in customer_grid_flat table.

That way, those attributes appear in Account Information tab of customer administration pages. However, when I try to save a data in them, data are not actually saved.

Customer attributes are note created in customer_entity table. Could that be the reason of why the attribute data are not saved? When creating custom attributes, should they be created in customer_entity table only? What is the goal of customer_grid_flat table?

Thanks