Hi Fabio,
Welcome to Affinity Forums :)
You have to "subtract" the inner circle from the outer one to create a "hole". To do this make sure the outer circle is below the inner one in the Layers panel, then select both circles and go to menu Layer ▸ Geometry ▸ Subtract.
Here's a clip (no sound) showing the process. Here instead of using the subtract menu command i've used the icon in the main toolbar instead. Note that if you press and hold ⌥ (option/alt) while clicking the subtract icon you create a compound object instead, that is, both the outer and inner circles will be preserved and can be edited independently even after performing the subtract operation (just go to the Layers panel and click the small arrow near the thumbnail to expand the compound layer and access the original circles). If you do not press ⌥ (option/alt) then the subtract operation will be destructive and you will not have access to the original shapes anymore, only to the resulting shape (that's what i've done in the clip below).
