So, I realized that FeatureLayer.selectFeatures does not adds the default lightblue highlight, you need to set the layer's selection symbol, so I copied the popup's one so that it looks the same :
So, I call the above before calling the selectFeatures method, problem is, instead of adding a symbol on top of layer symbol, it substitutes for this one, so it ONLY show the lightblue highlight border.
My question is, how can I have the same behaviour as when I click a feature? My guess is that the Popup adds the selection symbol on the Map graphics layer on top of the clicked feature, but can I do this without having to manage graphics added to another layer?
But can I do this without having to manage a new layer?
You will have to maintain a new layer. There is not a way to do this without a new layer that I am aware of, without editing the originals layers symbology for the the selected feature.
How about asking the Popup to "select" multiples features at once, is this possible?
Nope. The popup will only select one feature at a time.