I'm the dev of SotV and I've been playing with Koikatsu and Charastudio for about a year now. Hopefully I can answer some of your questions.
Assets like backgrounds and items are mostly from mods or just scenes people share online(Better Repack contains A LOT of essential mods and plugins and Koikatsu Discord is also a good place for scenes, characters, texture overlays etc. and knowledge).
Stuff like glowing eyes... There are several ways to do achieve this effect. Some people use Colored Shadow item and place it on the eyes. If you've ever played Corrupted Kingdoms, the dev uses this method for mind control scenes. I'm not a fan of it myself but to each their own.
For my game I use special shader for iris - in Charastudio pick character, select MAterial Editor, look for hitomi(iris) and switch to shader with Emision(I use StandardMDK) and you will need to manually tweak the colors, the strenght of Emission and some other stuff like LightCancel).
You can also use KKPE plugin > Adv Mode > Bones >hitomi and tweak postiton and rotation of the iris to achieve crossed eyes effect.
The final outcome looks something like that
ibb.co/16QhHnY