No, the games do not share developers most likely. It is common for games like these to purchase asset packs instead of making everything themselves. These packs are then also bought by other devs for their games. This is why you will see familiar scenes in Renpy games.
koikatsu was the og, all of theses more 3d anime type games use it to pose/make scenes and make characters. you can tell by the backgrounds they use/outfits/hair being the same in a lot of games. Also if you are just a koikatsu player you can recognize it anywhere. They should start saying that they made it with koikatsu especially if they want people to buy their games or support them on patreon.