It definitely is graphic settings ingame as well, you obviously can't achieve the same image quality on lower graphic settings that higher settings can
It also depends on the visual mods that you have installed. As nice as the vanilla graphics are they don't even come close to the visuals that VisualV (in my opinion) for example achieves, so having any visual mod installed whether it's VisualV or VisualV + NaturalVision Remastered (haven't tried any other ones so can't really recommend them) does help a lot
Also another thing to note is that at least from my understanding the way the Rockstar Editor works (which I use to take the screens) is that you can record clips on for example medium settings and then increase your settings to ultra before you start editing the clips in the Editor. That way you can still comfortably play the game, but also take nice screens afterwards
As for the vehicles, the older ones such as the SAHP Premier Classic & Ruiner are made by 11john11, some of the more modern ones that I've used have been by IlayArye and Vx5 Voltage and some were from the Realism Dispatch Enhanced mod. They along with plenty of other people have made some really nice lore-friendly emergency vehicles that can be used with LSPD:FR 😁