Illusion How to make/design my own interiors and such using assets from the game?

[Grato Nite]

I'm still new the game/software (lol.) the thing is, where can i design my interiors or exteriors using base assets? there's like bunch of exes, and i still don't know each's function and use.
It'll be helpful if someone can tell me which exe is for what

Also what's the best .exe for making CGs for visual novels?