Most VN's that use Koikatsu will have similar assets. The vast majority of people use the better repack, which includes a large selection of premade mods and assets. If you play enough you will see similarities between them.
Koikatsu is what the characters are made in. IT's a dating/h-game but includes the ability to make custom characters. It also includes Charastudio, which is where all the renders are made with the poses, animations, scenes, etc. If you search Koikatsu Party on steam you can see the steam version of the game.