I don’t think you can have a separate C folder. The few places I see cgo being used, it appears to be a combination of .c files and “inline” C code in comments above the import "C" statement in .go files. For example, two that I found after Googling for “example cgo projects”:
If I may make an observation: In your first question here, you said that you are working on your first project and it is “in Go,” yet all of your questions have been about how to reference C or C++ code from Go. What exactly is this C/C++ code? Is it really worth a month’s worth of effort to figure out how to use it from Go instead of just learning and rewriting it into Go? The answer might be “yes,” of course, but I figured I’d ask. Most of the answerers on this forum are more inclined to answer questions about how to do things in pure Go and not cgo.