In the pictures above I’m on my back under the tail pipes, shooting a picture of the underside of the left tail light and finisher assembly. The finisher being part #4 in the picture in your first post. The finisher should have a small screw hole that allows that corner of the finisher to be attached to the outer edge of the sheet metal tail light housing.
The picture below should clarify the positioning. I added an arrow pointing to the position of the phillips screw you are asking about.
These two pictures may also help. They show the bracket that is welded to the body that the bolt passes through to go into the captive nut on the finisher. The bolt and the thick square washer go on the inboard side of the bracket. You’ll not the hole the bolt goes through is square and slotted so you loosely assemble the bolt and finisher, slip it into position and then tighten the bolt to hold the finisher firmly in position. The yellow arrows point to the hole.