To know where you can check your detailed breakdown of all your charged fees, you can access the cost table report for your Cloud Billing account. In the Cost management section of the Billing navigation menu, select the Cost table.
Regarding why the charges were made even if you opted for free services is so you can better understand the costs of using Google Cloud after your free trial ends. As you notice in your provided screenshot, the Subtotal and Total is R$ 0,00.
In the sample report below, your usage costs appear in the Cost column, free trial credits appear in the Discounts column and Promotions and others column.