Hey, thanks for the kind words, and I'm glad you're enjoying the game! :)
As for your questions, well, feel free to ask anytime - I'm always happy to help players out if I can!
So, as to what I use, here's all the tech:
Software Development Kit (SDK): Ren'Py. The best free SDK for making visual novels, although if you want something more along the lines of a point-and-click then I can highly recommend Visionaire Studio.
Character Design/Modelling/Scene & Anim Creation: Koikatsu with all DLC and every mod I can get my hands on. Notable mods: Video Export, Screenshot Manager, Lighting and Timeline.
Render Editing: Photoshop (Although Krita is an amazing free alternative - I'm just used to PS's text options)
Animation Editing: Premiere Pro (Video Export mod will handle 99% of your animation needs, though. I just use Premiere to apply black bar overlays and to properly loop effects like water. This one isn't really necessary, honestly, if you're smart about camera angles.)
Music & Sound Design/Editing: Audacity. Mostly used for equalising the dB levels (effectively the baseline and capped volumes) of music so that players don't have to keep adjusting their volume control. Also used to edit and create certain sound effects used in-game. Free, and highly recommended. Good music/sound is half the battle when it comes to making decent games.
Workflow Management: Codecks. This isn't really necessary, but if you're looking for a good, free, workflow management tool and Notepad files are getting too out of control then I highly recommend this one.
And that's basically it! I might be forgetting one or two minor things but those are the tools I use to create CK.
As for how to build monster girls, well the best advice I can give is to get very familiar with bone modding and accessories in the character creator, and then analyse other people's creations (such as the ever-amazing Zolla) to see how they did what they did. Unfortunately there's no "magic bullet" for building Monster Girls, though, and Koikatsu has no real collision detection so clipping will occur.
Good luck! And when you start your project be sure to head over to the CK Discord and share your work on #i-made-this and #link-sharing! :)