Information on KSP IVA modding tend to be scarse, or old, so this is an attempt at making a centralized place for that, with all my experience and others's I could find, put together.

If you have a suggestion, for changes, errors / mistakes, or additions to the IVA modding Wiki, feel free to write about it in the Discussions section > Wiki-feedback channel

There is also a KSP forum thread, for discussions, suggestions and questions : Complete IVA modding guide : modifying, creating - Unity, Blender
Alternatively, you can join my KSP discord server and ask your questions there : https://discord.gg/GcfDk4DYHU

• Terminology
• Modifying existing IVAs
• Creating your own IVAs
• Unity editor setup for KSP
• Useful links and resources

IVA : Intra-Vehicular Activity, refers to the actual action of being in first-person view in the internal space, but it is also always used to refer to the internal models + their props themselves (the entire thing)
Internal : the internal model .mu, often found in the /Spaces folder, in mods
Prop : object placed in the internal through the Unity Editor (99.9% of the time), that the player interacts with, or not (decorative). Usually found in /Props folder, in mods
RPM : Raster Prop Monitor mod
MAS : MoardV Avionics Systems mod
"UNITYGamedata" : This is a term only valid in this wiki guide, for ease of understanding and to avoid confusions. It refers to the Gamedata folder you need to setup, to use with Unity, see 4.2 KSP Gamedata setup for Unity