How to overlay a vector file (lines or polygons) to a S2 image?
Actually there are two ways.
One just for the current view.
Open the Layer Manager (It’s on the right side). Click on the + sign and then select one of the options (e.g. ESRI Shapefile).
This layer will then only be visible for this image view.
You can also go to File -> Import -> Vector Data -> ESRI Shapefile in the menu.
This shapefile will be loaded into the product. You can then selected the imported shapefile among the geometries in the Layer Manager.