J channel is not flashing, it is for the siding to set into. Basically the wind has less chance of grabbing the siding to rip it off. However, J channel can cause issues. But by the look of your pictures, the issue is someone put at least a second layer on the roof and didn't rework any flashing.

I'm not seeing apron flashing on the front of dormer either. The step flashing should be worked so it overlaps the apron flashing. Someone went over top of the apron flashing. Bad idea, it's a water trap.

Here's a link to show you the basics.

Roof flashing corner post

Bad flashing detail. Flashing runs up underneath the siding and should be visible above the shingles at the bottom wall. Weaving the flashing into the shingles going up the sidewall appears to be the best way. If the roofer didnt remove the siding to apply flashing they didnt do it right. This is the main reason I like simple roofs, fancy dormers and such various architectural flourishes are a PITA for roofers, and will cost you more for roofers that know how to do it because they know it is PITA to do it right. Otherwise cheap roofers will do it wrong majority of the time.

http://www.finehomebuilding.com/2006/11/01/installing-step-flashing

For others in the future.

https://www.familyhandyman.com/list/vinyl-siding-installation-tips/