Front winshield seal at top coming apart
 

How/Who will repair the seal on the windshield?

Welcome- go to a local windshield replacement service to get an estimate on the rubber seal.

2nd what dk said - take it to a windshield shop. I had an issue a few years back where the rubber seals on the drivers side was starting to come off at the top, tech came out with a silicone gun, slapped it back in there on the spot and sent me on my way free of charge.

Front wondshield
 

My problem is I just purchased the car and it's totally missing. There is a gap between the glass and the top edge of the car. Not leaking but just looks rough.

You are probably missing this: https://www.nissanpartsdeal.com/part...720_A001,72811
Check it out.
// Al
https://cimg7.ibsrv.net/gimg/my350z....da72449a3c.jpg

Windshield trim
 

Any idea how this attached to body of car??

From what I've read, the windshield will have to be removed to replace this piece.

It seems to go between the windshield and the top trim. I'm not an expert, but looking at mine it seems like you can take off without removing the windshield.
Br,
Al