Well we definitely all use Koikatsu (or Koikatsu Party depending on your location) along with the many, many user-created mods there are for it (almost certainly fromm ManlyMarco's mod patch).
After that, it changes from creator to creator. I can only speak for myself from this point on, but in addition to KK I also use:
1. Photoshop (for editing the rendered images - resizing, touching up, or adding/removing elements)
2. Premiere Pro (to create looped animations where the VideoExport mod doesn't work, and to apply edits and overlays where necessary)
3. Audacity (to edit all audio files to the same decibel level, as well as create my own sound effects where necessary)
I also use Notepad for making story/mechanics notes, Discord for community engagement and livestreams, and an external SDD drive to backup all my data in case of catastrophic computer meltdown (learned that one the hard way!)
Hope that answers your questions, but if you have anything else you'd like to know, feel free to ask!
And good luck with your project! Be sure to show it off on the CK Discord under the #i-made-this and #link-sharing channels once you're ready to let everyone see it! :)