In QGIS I don't know how to crop an area of geologie within the boundaries of my studied area, which is UAT_Iasi.
Can you please give me some suggestions?
You want the Clip tool, accessible either from the menu under Vector/Geoprocessing Tools or in the Processing Toolbox under Vector overlay.
Set the Input layer to the layer to be cropped (geologie
in your case) and Overlay layer to the layer that specifies the outline (UAT_lasi
in your case):