Almost certainly in-engine. At this point there's not much reason to do offline renders without using your engine if you're a studio like Rockstar - you already

Almost certainly in-engine. At this point there's not much reason to do offline renders without using your engine if you're a studio like Rockstar - you already have tools for building cinematics and your game engine can scale quality and level of detail, etc. The same people who do your in-game cinematics can (if you choose to go that way) apply their talents to your trailers. GTA5 has built in recording and editing functionality too, so things like adjusting camera angles after you've recorded a sequence are trivial. I have no doubt all the same tooling is available to the GTA6 dev team, even if the game won't ship with it accessible to players. Of course once you've got footage out of engine, it's probably spliced together with transitions and voiceover in Adobe Premiere or Davinci Resolve, but that's pretty normal.