Hey there, It's not a secret, haha. We just use the build in animation program from Unity. For some assets we use the bones tool (the boobs and dicks for example)
We either rig the characters with the bones tool and use IK-chains or we just parent all the assets to each other. We don't use any special plugins to make this game. Just the build in tools that Unity provides. We do have more then 10-15 years of animation experience.
The best advice that I have ever gotten about animation. Is to animate something with a purpose or reason. Do not just animate something because it has to move. That will make your animations boring. So for example. If you make a walk cycle. Then keep in mind why that character moves like that. Instead of only animating it because it has to move.
If you want more advice then you can send me a message on Twitter.