It depends on the type of content you want to create.
I think it also depends on what you mean when you say "to use in-game built engines".
You aren't going to be able to create a full game in Sims or GTA. You can develop mods which add adult content to both.
It sounds like you are asking how to create images which can be used in a game. Taking screenshots or rendering images which are then shown in a AVN style game.
Typically, images are created in a graphics program. That can be anything from AI, Daz3D, Blender, HoneySelect or even MSPaint. Those images
can look vastly different and so it really is up to you (and your graphics skills).
The game engine is usually how those images are shown to the player. So engines like RenPy, RPGMaker, Twine, HTML, Unity, Unreal3d are all viable options. (Which engine will depend on your programming skills or ability to learn a programming type skill).
It isn't that clear from your message if you are talking about "how to create images?" (Sims/GTA/Daz3D/etc) or "how to create a game that shows those images" (RenPy/RPGM/HTML/etc). Or perhaps both?
If you are talking about developing a game in Unity or Unreal3D, it might also be that you won't be using existing images and so instead you'll need 3D models, meshes and all the other realtime 3D stuff.
