Sealing metal siding against window j/c channel?
I'm not a professional but I've replaced a few siding panels. In all the houses I've lived in (US), there wasn't any insulation or seal in the J or C channel. The one thing I wonder about is the colors. All of them I've seen the color matched the siding making gaps even less noticeable, I don't know if your channel actually has an orange interior. If there were a seal to be concerned about, it would be between the channel and the wall.