I know blitz3d has a few different coding languages you can choose from. Do you guys use the BASIC language, C++, C#, etc? Also, is it all scripting, or does it have an object interface (basically like unreal engine and such where you can add models without having to script it in), or do you have to basically write everything from code. I want to make mods for this, so I am curious as to what you guys use.

Blitz3D (at least the version used for CB) only supports its own, very limited language Blitz Basic. There is no kind of visual editor, everything is written in code.
Personally I would advise against modding, as SCP: CB is a very archaic game with an evershrinking playerbase.